Surface Review and Letters

Core level spectroscopy offers probing of atomic matter localized both in time and space. The temporal localization is due to the short lifetime of core holes, which opens a window from the energy domain to the low femtosecond time domain. The thus enabled studies of dynamic phenomena are exemplified by ultrafast dissociation of molecules. It is shown how information may be obtained about a characteristic dissociation time scale and the fragment velocities. The latter are also shown to be photon-energy-dependent. The spatial localization is due to the atomic character of core levels even in polyatomic systems. Clusters have a high and size-dependent fraction of atoms at the surface, and this leads to size-dependent properties. The spatially local character of core level spectroscopy is shown to be very valuable for separating contributions from surface and bulk atoms in free clusters.

The effects of interaction between the vibrational and electronic structure, so-called vibronic coupling, are reviewed in the context of core-ionized states. High resolution C 1s spectra of ethane and deuterated ethane are used as an example to compare our reasoning with measurements.

Recent studies on nuclear dynamics of core-excited and Auger final states are reviewed. Emphasis is given to the nuclear motion of the core-excited states and its influence to the subsequent decay and fragmentation processes. As specific examples we discuss two-dimensional nuclear motion caused by the O 1s → 2b₂ excitation in H₂O and by the C 1s → 2π_u excitation in CO₂, as well as out-of-plane motion caused by B excitation and asymmetric stretching motion caused by the F 1s ionization in BF₃.

Despite their importance to the photochemistry of the terrestrial atmosphere, and many experimental studies, previous characterization of the Schumann–Runge (SR) bands of O₂, (1750–2050 Å) has been limited by poor experimental resolution. In addition, our understanding of the SR spectrum is incomplete, many rovibrational transitions in the perturbed region of the spectrum [B(v > 15)] remaining unassigned. We review new very-high-resolution measurements of the O₂ photoabsorption cross section in the SR bands. Tunable, narrow-bandwidth background vacuum-ultraviolet (VUV) radiation for the measurements (~ 7 × 10⁵ resolving power) was generated by the two-photon-resonant difference-frequency four-wave mixing in Xe of excimer-pumped dye-laser radiation. With the aid of these cross-section measurements, rovibrational and line-shape analyses have led to new insights into the molecular structure and predissociation dynamics of O₂. The current VUV laser-spectroscopic measurements are shown to compare favourably with results from two other very-high-resolution experimental techniques, namely laser-induced fluorescence spectroscopy and VUV Fourier-transform spectroscopy, the latter performed using a synchrotron source.

The changes in energy and photoionization spectra of atoms upon confinement by a spherical environment are explored theoretically. Two kinds of confinement are considered: an endohedral confinement, such as inside the bucky-ball C₆₀, and an impenetrable spherical confinement of adjustable radius. We demonstrate modifications in the energy spectrum and electron correlation effects in confined atoms, the appearance, nature and origin of "confinement" resonances in photoionization spectra of such atoms, as well as new regularities in the periodic table for "compressed" atoms. These findings are of importance for basic and applied physics and chemistry of atoms, molecules, surfaces, etc.

The spectra of photon yield from solid Xe and Kr were measured in the energy range of absorption of Γ(3/2) n = 1 excitons. Using combination of time-resolved spectroscopy with selective photoexcitation by synchrotron radiation, the creation of free excitons was experimentally separated from direct population of molecular emitting centers. For the first time the threshold of photon absorption by molecular trapped centers is observed in excitation spectra of free exciton luminescence.

Angle-resolved resonant Auger spectra for the transitions Xe are recorded with optical and electron energy bandwidths smaller than the lifetime widths Γ of the Auger initial and final states. All the multiplet structures are resolved for the ionic core 4d^-2 of the spectator Auger final states and assigned as 4d^-21S₀np, ¹D₂np, ¹G₄np, ³P_{0, 1, 2}np and ³F_{1, 2, 3}np, n = 6, 7, by comparison with previous measurements and calculations for the corresponding normal Auger emission. The resonant Auger anisotropy parameters obtained from the angular distribution measurements agree well with those expected from the spectator model.

The OI emission cross section and the absorption cross section of O₂ were investigated near the threshold for neutral dissociation into the OI state with a very narrow bandwidth (0.9 meV) of the exciting radiation. The fine-structure components of vibrational levels were resolved for the first time in the emission cross section. A certain degree of selectivity in the population of fine-structure components was observed.

Normal Auger electron spectra from Kr2p hole states, L₂₃M₄₅M₄₅ and L₂₃M₂₃M₄₅, have been measured using monochromatized synchrotron radiation and a high resolution electron spectrometer. Measured spectra were reproduced with a fitting calculation, where Voigt functions including an instrumental resolution and natural lifetime widths of related core hole states were used. At most final states estimated energies for spectral peaks agree with those by electron and ion beam techniques. Relative intensities for some peaks are appreciably different from the previous results.

SF₂ radicals were generated by a pulsed dc discharge in a gas beam of SF₂ and Ar. The (2+1) resonance-enhanced multiphoton ionization (REMPI) excitation spectrum of SF₂ radical was obtained between 330 and 365 nm. The SF⁺ cation signals were also observed in the same wavelength range. Ab initio calculations show that the C state is a predissociative state with Rydberg characteristics. The REMPI spectrum is assigned to a two-photon resonant excitation of the SF₂ radical (the B¹B₁ and C¹A₁ states). Potential energy curves of excited states of the SF₂ radical obtained by ab initio calculations suggest that SF⁺ cations originate from the ionization of SF fragments yielded from the dissociation of the C state.

The desorption of fragment ions from sulfur-containing amino acids following the sulfur K-edge photoexcitation has been investigated using monochromatized synchrotron radiation. The samples investigated are pellets of neutral L-cystine, L-cysteine and L-methionine. It was found that only atomic S⁺ ions are desorbed following the S 1s → σ* resonant excitations, while various kinds of molecular fragment ions containing carbon, nitrogen and oxygen atoms are desorbed by the low-energy electron irradiation. The photon-energy dependencies of the S⁺ yields approximately resemble those of the total electron yields (TEY's). On the other hand, the edge-jump ratios, defined as I_on/I_off, where I_on is the intensity at the core-to-valence resonance energy and I_off is that at off-resonance energy, for the S⁺-yield curves are by more than ten times higher than those of the corresponding TEY curves. The edge-jump ratios and the partial electron yield spectra revealed that the low-energy electrons produced following the Auger decays scarcely contribute to the ion desorption. Also, it was elucidated that the S* desorption is induced by the direct core-to-valence excitation which is followed by the S KLL Auger decay. It was concluded that the core-to-valence resonant photoexcitation and succeeding specific S–S and S–C bond scissions play a major role in the ionic fragmentation in the present amino acids.

Auger electron spectra were studied by scanning the photon energy near the L₃ threshold of krypton. Two resonant transitions were observed in the photon energy region 1673–1678 eV for the first time. They were identified to be the resonant 3d^-25s and 3d^-24d states originating in the 2p_3/2^-15s and 2p_3/2^-14d excitations, respectively.

Angle-resolved ion yield spectroscopy revealed that ionic fragmentation of a T_d symmetry molecule SiF₄ is anisotropic after F 1s photoexcitation. The anisotropic fragmentation is interpreted as being due to preferential bond rupture at the F 1s core-hole site. The probability of the bond rupture at the core-hole site is three times as large as those at other sites. The origin of the preferential bond rupture is discussed in terms of symmetry breaking due to dynamical core-hole localization.

The C 1s^-12π_u¹Π_u state of a linear molecule CO₂ splits into two states. The lower state becomes bent and the upper state remains linear. As a direct probe of this Renner–Teller effect, the bent and linear geometries of the Renner–Teller pair states have been probed by means of triple-ion-coincidence momentum imaging.

High resolution near-edge excitation of isolated molecules and homogeneous molecular van der Waals clusters is reported. Resonant excitation near the element K-edges of isolated and clustered carbon monoxide and nitrogen gives rise to small spectral shifts that can only be detected by high spectral resolution. The experimental results indicate for the vibrationally resolved 1s → π* transitions of isolated molecules that symmetric line shapes cannot be used rather than the lifetime-broadened rotational fine structure. Clustered molecules exhibit characteristic redshifts of the order of a few meV relative to the corresponding isolated molecules. Changes in Franck–Condon factors and changes in spectral line shape are observed, as well. The experimental results are discussed in the framework of the quasiatomic approach with respect to intermolecular interactions and dynamic localization of resonant core-to-valence excitations.

The single-photon ionization mass spectrometry of primary photofragments of chlorine monoxide (ClO) is reported. The use of laser-produced plasma (LPP) radiation in pump–probe experiments allows us to ionize the laser-generated atomic photoproducts with tunable radiation in the vacuum ultraviolet regime (E = 10–18 eV). Autoionization resonances of the atomic photoproducts are used to establish the quantum states of the photoproducts and to enhance the detection sensitivity. Predissociation and direct dissociation of A(²Π)-excited ClO yields Cl(²P)+O(³P) as well as Cl(²P)+O(¹D), respectively. The quantum yield of O(¹D) formation is reported in the threshold regime near the ClO [A(²Π)] dissociation limit.

Energy relaxation processes of photoexcited Ar_M clusters (M < 50) covered with a shell of Kr atoms (up to 30 atoms) which are embedded in large Ne_N clusters (N = 3500) are investigated with energy-resolved fluorescence spectroscopy. In the energy range of the Ne cluster absorption (16.5–18 eV) a strong energy transfer to the embedded Ar subcluster is observed, which results in desorption of electronically excited Ar* atoms. Ar* move through the Ne cluster, desorb and emit visible and near-infrared light in the vacuum (4p → 4s). By coating the Ar clusters with Kr atoms, the Ar lines disappear and 5p → 5s transitions of Kr become dominant. Additionally, new bands occur, which are assigned to transitions of perturbed atomic Ar 4p states inside Ne clusters. A simple kinematic model is suggested to describe the suppression of desorption of excited Ar atoms from Ne clusters.

Electron–ion coincidence measurements with energy-resolved electrons are a powerful tool in studies of molecular fragmentation processes, since fragmentation from a specific doorway state can be monitored while the remaining reactions are discriminated. Presently, we have applied this technique in measurements of coincidence spectra of the CD₄ and SF₆ molecules after valence and core electron excitation. The newly constructed energy-resolved electron–ion coincidence station developed for this project has been used. Our results on CD₄ are the first demonstration of a correlation between vibrations and molecular dissociation observed in this molecule. The SF₆ spectra reveal strong selectivity in dissociation following distinct electronic states of the molecule. This selectivity reflects the bonding properties of the potential surfaces involved in the studied processes. The obtained results exemplify the potential of the new multicoincidence station used in this work.

Excited states of ozone are investigated based on the absorption spectrum measured in the wavelength region 105–200 nm. Two Rydberg series corresponding to the transition 6a₁ → ns, np and 4b₂ → ns and np were assigned for the most of the strong peaks in the absorption spectrum. In 112–105 nm wavelengths, the progression of vibrational bands with an energy spacing of about 622 cm^-1 was measured which was unknown before.

A recent paper (Science2000, 289, 611) has suggested that an anthropogenic greenhouse gas, SF₅CF₃, recently detected in the atmosphere, has the highest radiative forcing of any gas-phase molecule. The ground state of is repulsive in the Franck–Condon region, the parent ion is not observed, and the onset of ionisation can only give an upper limit to the energy of the first dissociative ionisation pathway of . Using a variation of threshold photoelectron photoion spectroscopy, we have determined the kinetic energy released into the two fragments over a range of energies. Using an impulsive model, the data has been extrapolated to zero kinetic energy to obtain a value for the first dissociative ionisation energy for SF₅CF₃ of 12.9 ± 0.4 eV. The enthalpy of formation at 0 K of SF₅CF₃ is determined to be -1770 ± 47 kJ mol^-1, and the dissociation energy of the SF₅–CF₃ bond at 0 K to be 392 ± 48 kJ mol^-1 or 4.06 ± 0.45 eV. The implication of this bond strength is that SF₅CF₃ is very unlikely to be broken down by UV radiation in the stratosphere. A similar experiment for , , and yielded values for their dissociative ionisation energies of 14.45 ± 0.20, 13.6 ± 0.1, 14.1 ± 0.5 and 14.5 ± 0.6 eV, respectively. The first two results agree with previous data on the CF₃ and SF₅ free radicals. The final two results yield 0 K enthalpies of formation of and to be 166 ± 52 and 4 ± 62 kJ mol^-1, respectively.

We study the linewidths in cascade Auger spectra in synchrotron radiation experiments and discuss their dependence on the lifetimes of the discrete states and the experimental conditions. In particular we consider the possibility of manipulating the linewidths of the first step decay by tuning the bandwidth of the incoming photons. This may be used as an identification method in cases where the spectra from different decays are energetically overlapping. As an example we take the cascade process in Xe following the 4d_5/2 → 6p resonant excitation.

Fluorescence spectra have been recorded in the wavelength region 300 nm ≤ λ(fluo) ≤ 500 nm after resonant 2p → 3d excitation of atomic Ar using monochromatic synchrotron light. The radiative emission is characterized by strong transitions from Ar⁺3p⁴ (¹S, ¹D, ³P) nℓ ionic states. It is mainly assigned to processes of the type 3p⁴4d → 3p⁴4p and 3p⁴4p → 3p⁴4s. In addition, we have analyzed the degree of polarization of the fluorescence lines and we have determined the alignment of the Ar⁺ ions, which are produced after the resonant Auger decay. In comparison to investigations based on photoelectron spectroscopy, the spectral analysis of the fluorescence provides complementary information about the electronic relaxation during the resonant Auger process and, in particular, it allows one to examine directly the remaining ion.

Photoionization to the A ²Π_u state and to the X ²Π_g state is studied using photoelectron spectroscopy. The experimental vibrational branching ratios are obtained for the first time in the 19–34 eV region for the v′ = 0–3 levels, for both molecules. Ab initio many-body perturbation theory is used to calculate branching ratios for ionization to the A ²Π_u state and branching ratios, vibrationally resolved partial cross sections and total cross section for ionization to the X ²Π_g state. The Franck–Condon breakdown observed in the photoionization of the N₂ 1π_u electron is mainly explained by autoionization from Rydberg and valence states of N₂, whereas photoionization of the O₂π_g electron is not fully explained either by channel coupling effects or by autoionization from known Rydberg series and valence states.

Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been used to investigate the decay dynamics of the valence electronic states of the parent cation of several hydrofluorocarbons (HFC), based on fluorine-substituted ethane, in the energy range 11–25 eV. We present data for CF₃–CHF₂, CF₃–CH₂F, CF₃–CH₃ and CHF₂–CH₃. The threshold photoelectron spectra (TPES) of these molecules show a common feature of a broad, relatively weak ground state, associated with electron removal from the highest-occupied molecular orbital (HOMO) having mainly C–C σ-bonding character. Adiabatic and vertical ionisation energies for the HOMO of the four HFCs are presented, together with corresponding values from ab initio calculations. For those lower-energy molecular orbitals associated with non-bonding fluorine 2pπ lone pair electrons, these electronic states of the HFC cation decay impulsively by C–F bond fission with considerable release of translational kinetic energy. Appearance energies are presented for formation of the daughter cation formed by such a process (e.g. CF₃–CHF⁺), together with ab initio energies of the corresponding dissociation channel (e.g. CF₃–CHF⁺ + F). Values for the translational kinetic energy released are compared with the predictions of a pure-impulsive model.

We report the Near Edge X-ray Absorption Fine Structure Spectra (NEXAFS) of a series of oxygen-containing organic molecules, namely formaldehyde, acetaldehyde, acetone, formic acid, methanol and dimethyl ether (DME), measured with high resolution at the carbon and oxygen edges. A vibrational progression has been observed at the oxygen 1s → π* resonance of formaldehyde, indicating that this state is bound with an excited state C=O stretching frequency of 136 meV. The spectra are compared with previous measurements and the applicability of the chromophore concept is tested for the functional groups present in these molecules.

The N = 2 double excitation series of helium was examined with the high performance U125 plane grating monochromator at BESSY II set to a resolving power of 90 000. A hitherto unresolved beat structure in the principal series was observed at very high excitation levels. The beat can be explained by the mixing of two series converging to different ionization thresholds and having an energy spacing corresponding to the energy split between the 2p_1/2 and the 2p_3/2 level of singly ionized helium. This indicates that the electron interaction at high excitation levels must be described within the JK coupling scheme.

The magnetic domain structure of patterned permalloy films and of a Co(0001) single crystal surface are studied with elliptically polarized light from the new nanospectroscopy beamline at ELETTRA in a low energy electron microscope, using it as a diagnostic tool in the commissioning phase of the beamline. Mirror and low energy electron microscopy as well as low energy electron diffraction are shown to be valuable fast techniques for system alignment and specimen characterization.

Soft X-ray microscopy has resolved 30 nm structures in biological cells. To protect the cells from radiation damage caused by X-rays, imaging of the samples has to be performed at cryogenic temperatures, which makes it possible to take multiple images of a single cell. Due to the small numerical aperture of zone plates, X-ray objectives have a depth of focus on the order of several microns. By treating the X-ray microscopic images as projections of the sample absorption, computed tomography (CT) can be performed. Since cryogenic biological samples are resistant to radiation damage, it is possible to reconstruct frozen-hydrated cells imaged with a full-field X-ray microscope. This approach is used to obtain three-dimensional information about the location of specific proteins in cells. To localize proteins in cells, immunolabeling with strongly X-ray absorbing nanoparticles was performed. With the new tomography setup developed for the X-ray microscope XM-1 installed at the ALS, we have performed tomography of immunolabeled frozen-hydrated cells to detect protein distributions inside of cells. As a first example, the distribution of the nuclear protein male-specific lethal 1 (MSL-1) in the Drosophila melanogaster cell was studied.

Soft X-ray microscopy allows one to study nanoscale heterogeneities in dry and wet environmental science, biological, polymer, and geochemical specimens. Recent advances in instrumentation at the X-1A beamline at the National Synchrotron Light Source at Brookhaven National Laboratory are described. Spectromicroscopy data analysis methods including component mapping and principal component analysis (PCA) are then discussed.

Scanning transmission X-ray microscopy (STXM) and X-ray photoelectron emission microscopy (X-PEEM) provide quantitative chemical analysis at a spatial resolution well below 100 nm. Soft X-ray absorption or near edge X-ray absorption (NEXAFS) contrast provides sensitive differentiation of species which have similar elemental composition but are chemically distinct. Due to the ability of soft X-rays at wavelengths below the O K-edge to penetrate water, and on account of lower radiation damage, soft X-ray microscopy is an ideal tool for providing quantitative information about soft matter in the context of biological, polymer and environmental studies. Examples are given from recent studies of: phase segregation in polyurethanes and polymer blends, protein adsorption on polymers relating to biomaterial optimization, and metal mapping in biofilms. These examples show that it is indeed possible to get quantitative (hard) information by combining soft X-rays and soft materials.

The development of high brilliance X-ray sources coupled with advances in manufacturing technologies of focusing optics has led to significant improvements in submicrometer probes for spectroscopy, diffraction and imaging applications. For instance, X-ray microscopy in the 1–10 keV energy range is better-suited for analyzing trace elements in fluorescence yield. This article will be biased towards submicron fluorescence microscopy developed on the ID21 beamline at the ESRF. The main technical developments, involving new focusing lenses or novel phase contrast method, are presented. Strengths and weaknesses of X-ray microscopy and spectromicroscopy techniques are discussed and illustrated by examples in biology, materials science and geology.

Adapting classical spectroscopic methods to the new challenge of studying nanomaterials, imaging techniques are the trendsetter in recent years. Among them is scanning photoelectron microscopy (SPEM) with submicron spatial resolution, where the sample surface is raster-scanned by a focused soft X-ray beam, and the emitted photoelectrons are collected at each point by the input optics of an electron energy analyzer. We have constructed such a station at SRRC in Taiwan, which is now fully in operation. In this paper, we introduce the specific features of the instrument and discuss application examples on the characterization of scanning-probe-induced Si₃N₄ to SiO_x conversion and electron- and plasma-induced chemical changes in alkanethiols self-assembled monolayers. In combining two-dimensional imaging and micro-photoemission, SPEM can reveal valuable information on the chemical and electronic properties of structured and multiphase materials.

The second development step of the SMART project, i.e. an energy-filtered but not yet corrected photoelectron emission microscope, operates at the undulator U49/1-PGM beamline at BESSY II. It already demonstrates the variety of methods of the final version: microscopy, spectroscopy and electron diffraction. Some recent experimental results are reported for these three operation modes. In addition, the theoretical improvement of lateral resolution and transmission of PEEMs in general by using an energy filter is discussed for systems without and with aberration correction.

Yttrium, lantanum and rare earth elements can be loaded with hydrogen inducing a metal–insulator transition and giving rise to optical switching from reflecting to transparent. We present angle-scanned photoemission experiments characterizing thin YbH_x films grown on W(110) at room temperature. Hydrogen loading is performed in an ultrahigh-vacuum-compatible high pressure (1 bar) reaction cell. Via full-hemispherical X-ray photoelectron diffraction and low energy electron diffraction, it is demonstrated that these films grow well-ordered and single-crystalline. Ultraviolet photoemission reveals a gap for the dihydride phase confirming a transition from reflecting to transparent as seen by visual inspection. Ion implantation through additional H+ sputtering allows one to increase the hydrogen content to x ≈ 2.4.

A novel X-ray technique for converting the phase information of weakly absorbing specimen into strong image contrast similar to Nomarski differential interference contrast (DIC) is presented. DIC for X-rays is accomplished by the fabrication of a novel X-ray optic (TZP) consisting of two zone plates (ZPs) on both sides of the same substrate, laterally shifted by about one outermost zone width. The feasibility of DIC for X-rays was proven at the ID 21 X-ray microscopy beamline at the ESRF using a full-field imaging microscope and a scanning transmission X-ray microscope, which were operated at 4 keV photon energy. In both microscopes, we observe a tremendous contrast enhancement of up to a factor of 25. Though first experiments were carried out at 4 keV photon energy, this X-ray DIC technique can be adapted to any photon energy where ZPs with appropriate parameters and imaging performance can be designed and manufactured.

A natural application of the emerging technique of photoemission microscopy to the study of semiconductor interfaces involves measuring a device in cross section to directly determine heterojunction parameters. We present here results on p–n GaAs homojunctions, which served as a prototype system to demonstrate the applicability of this technique to buried semiconductor interfaces. We also describe preliminary measurements of the electrostatic potential profile across Al/GaAs Schottky junctions.

Removal of Cu contaminants from Si wafer was carried out using remote hydrogen plasma (RHP) and UV/O₃ cleaning techniques. The concentration of Cu impurities on the wafer surface was monitored by TXRF (total reflection X-ray fluorescence) and XPS (X-ray photoelectron spectroscopy). Our results show that Cu impurities can be effectively removed by hydrogen plasma and UV/O₃ cleaning techniques, if it is performed under optimum process conditions. The optimum process parameters for the remote hydrogen plasma cleaning are the rf power of 20 W and the exposure time of 5 min. The optimum exposure time of the UV/O₃ cleaning for Cu impurity removal is 1 min. A two-step cleaning process composed of remote hydrogen plasma cleaning first and UV/O₃ cleaning next has been found to be more effective than a single UV/O₃ cleaning process, a single remote hydrogen plasma cleaning process, or a two-step cleaning process composed of UV/O₃ cleaning first and remote hydrogen plasma cleaning next. Cleaning efficiency is maximized at optimum process conditions where Cu contaminant removal effect and recontamination effect are traded off. Increasing the process parameters higher than the optimum values would decrease the cleaning efficiency.

The changes of the structure and chemical states of photoluminescent p-type porous silicon (PS) caused by annealing in vacuum were investigated with atomic force microscopy and X-ray photoemission spectroscopy. The relative intensities of the silicon dioxide and suboxide peaks increased with the annealing temperature. The average size of the fine crystallites of the as-prepared samples was 5–10 nm and became 50–100 nm after being annealed at 550°C. The cause of photoluminescence quenching upon annealing is discussed.

To analyze the oxidation states of the graphitic surface of microporous carbon, soft X-ray absorption spectra in the 0 K region have been obtained for microporous carbon and various aromatic compounds. The aromatic molecules studied are substituted with one or more of the following oxygenated functional groups: hydroxy (–OH), carboxy (–COOH), carbonyl (>C=O), formyl (–CH=O), and ether (–O–). From comparison of the soft X-ray absorption spectra of microporous carbon and of reference aromatic compounds, the most probable chemical bonding states of oxygen in microporous carbon are found to be –COOH and >C(H)=O. Spectral features in the soft X-ray absorption spectra of microporous carbon are well explained by the O2p density of states in these oxygenated functional groups from discrete variational (DV)-Xα molecular orbital calculations.

The spectroscopic properties of LiKGdF₅ crystals doped with Er³⁺ or Tm³⁺ and codoped with Dy³⁺, Tb³⁺ or Sm³⁺, and of LiKYF⁵ crystals doped with Er³⁺ or Tm³⁺ and codoped with Dy³⁺ have been studied under excitation by vacuum ultraviolet (VUV) radiation. It has been found that in LiKGdF₅ crystals doped with Er³⁺ or Tm³⁺ the VUV energy absorbed by 4f–5d transitions in the Er³⁺ or the Tm³⁺ ion is nearly completely converted into the UV/visible emission. At least in the LiKGdF₅:Er³⁺ crystal some part of such a conversion is due to the energy transfer from the 5d state of the Er³⁺ ion to the neighboring Gd³⁺ ion as a result of cross-relaxation giving rise to the emission of two UV/visible photons instead of one VUV quantum. LiKGdF₅ crystals doped with Er³⁺ and codoped with Dy³⁺, Tb³⁺ or Sm³⁺ can be considered as promising materials for phosphors excited by VUV radiation from noble gas discharge.

The catalytic properties of LaCoO₃ in the oxidation of organic molecules in aqueous solution are explored as a function of doping with both Sr substitution for La and Fe and Ni substitution for Co. VUV photoemission is used to explore the surface reactivity of the ceramic catalysts in aqueous solution, using H₂O as a probe molecule. These measurements are complemented by EXAFS and XANES measurements designed to probe the local defect structure and by GC measurements of catalytic activity in the aqueous epoxidation of crotyl alcohol. We relate the observed catalytic activity to the defect structure of the doped materials. In Ni-doped materials, oxygen vacancies appear to be the predominant defect, whereas in Fe-doped samples, electron holes are stabilised on Fe, leading to very different behaviour in oxidation. The surface reactivity to water is also influenced by the TM d electron count, with water binding more strongly to Fe-doped materials than to those containing Ni. The influence of these factors on the rate of the unwanted hydrogen peroxide decomposition reaction and hence on activity in epoxidation is discussed.

We investigated the various oxidation conditions of stainless-steel surface using synchrotron radiation photoemission spectroscopy. Stainless-steel samples are oxidized at 450–550°C in various oxygen partial pressures. Increasing the annealing temperature from 450°C to 550°C, the trivalent chromium concentration in the surface increased and iron oxides decreased. The PES spectra from stainless steels oxidized at 550°C show that there exists the critical oxygen partial pressure, Torr. Below this critical pressure, the oxide formed mainly consists of chromium oxide, while the oxide formed mainly consists of iron oxide above the critical pressure. The oxidation behavior is in good agreement with the Ellingham diagram (a free-energy/temperature diagram).

In this study we measured the Fe, Mn, and Ni L_2,3-edge X-ray absorption near-edge structure (XANES) and K-edge extended X-ray absorption fine structure (EXAFS) of the ferromagnetic (FM) NiFe and antiferromagnetic (AFM) FeMn bilayer films prepared with various annealing temperatures. The branching ratios of the white-line intensities in the Fe, Mn, and Ni L_2,3-edges XANES spectra and consequently the magnetic properties of these exchange-biased FM NiFe - AFM FeMn bilayers are found to depend strongly on the annealing temperature. We find that the first peak in the Fe, Mn, and Ni K-edge EXAFS Fourier transform spectra are very similar, which suggests that the nearest-neighbor bond lengths among Fe, Mn, and Ni atoms are essentially the same in the NiFe–FeMn bilayers. However, the peaks at distances greater than ~ 3 Å appear to be sensitive to the annealing temperature especially for the Fe and Mn K-edge spectra, which suggests that annealing alters the atomic structures of the next-nearest-neighbor and more distant shells surrounding the Fe and Mn atoms in the NiFe–FeMn bilayers.

The time-resolved emission spectra and the excitation spectra of the main emissions have been measured for the first time in stressed and unstressed RbI crystals using synchrotron radiation of 10–30 eV at 8 K. It has been shown that the applied elastic uniaxial stress (EUS) offers promise for the separation of the elementary multiplication mechanisms of electronic excitations. A different influence of EUS on the self-trapping of excitons or holes at 8 K has been discussed. The mean free path of free excitons decreases, while that of valence holes increases in a stressed RbI crystal.

Aminosilylated surface was treated with halide-substituted aromatic aldehydes, and the resulting molecular layers were examined with synchrotron X-ray photoelectron spectroscopy at the 2B1 SGM and 4B1 microscopy beamline in the Pohang Accelerator Laboratory. It was observed that the halide group of the film diminished upon the irradiation, but the other bands were constant in terms of the intensity and the shape. This observation indicates that the functional groups of the organic monolayers are cleaved selectively by soft X-rays. The cleavage rate was measured as a function of photon energy and normalized with photon flux. The cleavage is first-order to the concentration of the functional group. Its rate constant is sensitive to the molecular structure of the organic monolayers and the kind of substituents on the aromatic ring.

The Si L_2,3 and C K soft X-ray emission spectra of the interface of the Ti(50 nm)/4H-SiC(substrate) system, thermally annealed from 800°C to 1000°C and characterized by soft X-ray emission spectroscopy (SXES), revealed the formation of a reacted region composed of silicides with Ti₅Si₃ as the majority formed species and carbides in TiC-like bonding. Also, the photoemission electron microscopy (PEEM) imaging of Ti(10 nm) film on 3C-SiC surface during in situ heat treatment showed the formation of island structures (in ring clusters) at ~ 800°C.

We have performed a photoemission study on an organometal spin-crossover complex, [Fe(2-pic)₃]Cl₂EtOH, using the combination of synchrotron radiation and laser. The core-level and valence-band photoemission spectra were measured for high-spin, low-spin, and photoinduced phases. The N 1s and valence-band spectra showed remarkable changes due to the photoinduced phase transition, indicating that the electron charge was transferred from nitrogen to iron at the photoinduced phase transitions. It was also found that the valence-band structure of the photoinduced phase is very different from that of the high-spin phase caused by the thermally induced phase transition.

The nonradiative decay processes of the Ba-5p core exciton in BaF₂ have been investigated by using photoelectron spectroscopy, since the exciton peak is missing in the excitation spectrum of the Auger-free luminescence. The decay probability of the direct recombination process estimated from the constant-initial state spectrum and the absorption coefficient was found to be smaller than unity, indicating the existence of the other decay processes. It is suggested that the Ba-5p core exciton is relaxed by about 0.9 eV.

The electronic structure of CrAs thin films with nominal thicknesses of 2 nm and 30 nm were studied by angle-resolved photoemission spectroscopy using synchrotron radiation. The CrAs film of 2 nm is expected to form a zinc-blende phase which is predicted to have a half-metallic band structure by theoretical calculation. On the other hand, it is considered that the CrAs film of 30 nm is a polycrystalline compound judging from reflection high-energy electron diffraction patterns. Different valence-band photoemission spectra were obtained for the two films. The photoemission spectra of the CrAs film of 30 nm showed no band dispersion, whereas a band dispersion was observed in that of the CrAs film with the nominal thickness of 2 nm.

Ion time-of-flight (TOF) mass spectra of poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF) thin films near fluorine and carbon K-edges were observed. For PTFE thin films, peaks corresponding to F⁺, CF⁺, and appeared, while for PVDF F⁺ and H⁺ were mainly observed. They indicate that for PTFE the polymer chain (C–C bonds) as well as C–F bonds are broken by irradiation of photons near fluorine and carbon K-edges, while for PVDF the bond scission occurs mainly at the C–F and C–H bond. Partial ion yields (PIY) of these ions for PTFE and PVDF thin films show strong photon energy dependencies near fluorine and carbon K-edges. The excitation from fluorine 1s to σ(C–F)* is specially efficient for F⁺ ion production for both PTFE and PVDF.

Resonant inverse-photoemission spectra of NiAs-type MnTe have been measured with the tunable photon energy mode at kinetic energies of incident electrons (E_k) in the Mn 3p–3d excitation region. The peak structure at 2.9 eV above the valence-band maximum (VBM) is resonantly enhanced at E_k = 50 eV and is attributed to transitions to d⁶ final states with ⁵E and ⁵T₂ symmetries. The broad structure around 12 eV above the VBM in the spectrum at E_k = 46.5 eV and a shoulder on the higher energy side of the main peak at 2.9 eV in the spectrum at E_k = 50 eV is assigned to final states and d⁶ final states with ³Γ symmetries, respectively, where stands for a ligand hole and Γ an irreducible representation.

The efficiency of ablation induced in poly(methyl methacrylate) (PMMA) by single soft X-ray pulses emitted from Z-pinch and laser-produced plasmas was determined. X-ray ablation of PMMA was found to be less efficient than that of teflon (PTFE). Nonthermal effects of the radiation on the polymer structure play a key role in the mechanisms of the ablation, i.e. the ablation can be explained by the formation of radiation-chemical scissions of the polymer chain followed by blowoff of low-molecular fragment fluid into the vacuum. The most promising application of this phenomenon seems to be micropatterning/micromachining.

Relaxation processes and stability of charge centers in preirradiated doped rare gas solids were studied combining spectrally resolved thermally stimulated luminescence (TSL) and exoelectron emission (TSEE) techniques. The thermally assisted emission of electrons was found. Comparison of the yields of electrons and photons in VUV and visible ranges made it possible for the first time to discriminate between reactions of neutral species and charge carriers and find their interconnection. Ranges of charge center thermostability were found.

High-resolution, uniformly calibrated Fe 2p_3/2 absorption spectra of various Fe(II) and Fe(III) compounds with the metal atom octahedrally coordinated to atomic and molecular ligands are analyzed expecting changes in the absorption spectra due to differences in the formal valence state and in the character of the chemical bond. Particular attention is given to revealing spectral characteristics of the 3dπ–2π(π^*) charge transfer (π-back-donation effect) between the iron atom and cyanide ligands.

We have observed magnetic domains of mesoscopic magnetic structures of Ni by synchrotron radiation photoelectron emission microscopy using circularly polarized soft X-rays from the bending magnet. The mesoscopic magnetic structures of Ni were fabricated by electron beam lithography combined with a chemical liftoff process. The geometries of the Ni mesoscopic samples were square, triangular and hexagonal with the sizes of 5 and 10 μm. The magnetic domains structures were clearly observed by dividing the images measured at the Ni L₃-edge by the images measured at the Ni L₂-edge. The magnetic domains of the Ni mesoscopic structures are discussed with the results of magnetic force microscopy and micromagnetic simulation.

This paper presents temperature dependence of emission spectra (290 nm and 340 nm bands), excitation spectra (250–3000 Å), luminescence decay time as well as their excitation effect for CeF₃ crystals. The results indicate that energy transfer from Ce³⁺ to defect centers in the CeF₃ crystals was very efficient as temperature increased. The 340 nm emission band may originate from some defects near Ce³⁺, rather than Ce³⁺ perturbed defects.

WO₃ thin film surface chemical composition has been studied by means of high resolution soft X-ray photoemission spectroscopy. Valence band and W 4f core levels have been analyzed on different sample positions and high lateral resolution images have been acquired. The valence band spectra have shown for the first time a marked increase of the W 5d density of state at the Fermi level, indicating the presence of metallic tungsten on the surface. This has been confirmed by the W 4f signal, which presents both metallic and oxidized phases. For the first time, spectromicroscopy on core states has clearly evidenced the presence of nonstoichiometric areas and a spatial localization of W⁴⁺ states on the surface.

Multiple scattering calculations using the FEFF-8 code were applied to simulate the X-ray absorption spectra recorded at the nitrogen K edge of III-N compounds. The major features of experimental N K XANES, including polarization effects, are well reproduced by the calculations, as is demonstrated for GaN. Further, FEFF calculations have been performed to investigate N bombardment-induced defects in III-V semiconductor surface layers. Recently reported nitrogen ion bombardment-induced spectral features of the N K absorption edge may be related to implanted nitrogen molecules by comparing experimental and calculated N K XANES.

X-ray photoemission and X-ray photoabsorption were used to study the composition and the electronic structure of ytterbium-doped strontium fluoroapatite (Yb:S-FAP). High resolution photoemission measurements on the valence band electronic structure were used to evaluate the density of occupied states of this fluoroapatite. Element-specific density of unoccupied electronic states in Yb:S-FAP was probed by X-ray absorption spectroscopy (XAS) at the Yb 4d (N_4,5-edge), Sr 3d (M_4,5-edge), P 2p (L_2,3-edge), F 1s and O 1s (K-edges) absorption edges. These results provide the first measurements of the electronic structure and surface chemistry of this material.

Dielectric films on Si or SiC were investigated using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and ion beam analysis techniques, namely high and low energy ion scattering (RBS and LEIS) and narrow nuclear resonance profiling (NRP) combined with isotopic substitution. For the Si substrate, attention was focused on the thermal stability of materials candidate to replacing SiO₂ in Si-based microelectronic devices (so-called "high-k dielectrics"): Al₂O₃, ZrSi_xO_y, ZrAl_xO_y, HfSi_xO_y, and GdSi_xO_y. Mobility of different atomic species — especially Si and O — was observed at both the surface of the films and the interface with Si. Such atomic transport may have serious consequences concerning application of these materials in the microelectronics industry. For the SiC substrate, attention was focused on the initial stages of thermal oxidation in O₂, seeking an understanding of its poorer electrical quality as compared to SiO₂–Si. It was found that the initial oxidation products are silicon oxycarbides (SiC_xO_y), while for longer oxidation times a mixture of SiC_xO_y and SiO₂ is formed in the near surface region of the growing film. The composition of the near surface region of such thin films is very similar to that reported in previous investigations for the near interface region when thicker oxides films are grown on SiC.

High-flux soft-X-ray irradiation effects were investigated for the thick a-Si:H and μc-Si:H films, glass, MgF₂ and CaF₂ plates. The a-Si:H film of 7500 nm thickness has been burst, and its broken fragments flew off within several seconds. From the changes of Raman spectra before and after the irradiation and the film thickness dependence, it is thought that this is caused by rapid crystallization. SR-induced etching of glass, MgF₂ and CaF₂ plates has been observed. Photon energy dependence of the etching depth has been found. Exciting the Si K-shell is effective for etching the glass substrate. On the other hand, photon flux is important in the case of fluoride, and the maximum etching rate of CaF₂ is more than 1 μm/min.

Electronic structure of perfluorotetracosane [n-CF₃ (CF₂)₂₂CF₃; PFT], which is an oligomer of poly(tetrafluoroethylene) (PTFE), was investigated by angle-resolved photoemission spectroscopy (ARUPS) with synchrotron radiation. Theoretical simulations of ARUPS spectra were also performed by the independent atomic center (IAC) approximation combined with ab initio molecular orbital (MO) calculations. Measured normal-emission spectra for end-on oriented (the long-chain axis of PFT is perpendicular to the surface) films showed an incident photon energy dependence due to the intramolecular energy band dispersion. The energy band structure along the long-chain direction of PFT obtained from measured and calculated spectra is also discussed.

The morphology and chemistry of S-treated GaAs(001) surfaces have been investigated by using an atomic force microscope (AFM) and a synchrotron radiation scanning photoemission microscope (SPEM) for a sample (A) which was directly blown dry with N₂ after the S-treatment and for another sample (B) which was rinsed by DI water and finally blown dry with N₂ after the S-treatment. AFM and SPEM images show for sample A a very inhomogeneous surface morphology, while the surface morphology of sample B appeared to be very homogeneous. On the surface of sample A, an inhomogeneous distribution of particles was observed by AFM. Nearly all these particles could be removed by the water rinse. Laterally resolved core level spectra analysis shows that on both samples the S bonds mainly to Ga. Only traces of As–S and As–As bonds exist at the sample surface while no S–S bonds could be detected. From the analysis of the O 1s core level it can be deduced that only a small GaAs surface fraction is oxidized. Furthermore, O–S bonds are present at the surface. On the sample before the water rinse, the number of O–S bonds is laterally inhomogeneously distributed. The number of O–S bonds can be drastically reduced by the water rinse, which also results in a homogeneous distribution of those bonds. We deduce that the O–S bonds belong to sulfate SO_x deposits at the sample surface which are the particles that were imaged by AFM.

The interface of the tris (8-hydroxyquinoline) aluminum (Alq₃)/LiF/Al system was investigated with ultraviolet photoelectron spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and X-ray photoelectron spectroscopy (XPS). Strong chemical interaction was observed at Alq₃ on Al interface. By inserting a LiF layer, this interaction is suppressed and interface energy level alignment is changed to reduce a barrier for electron injection. The enhancement of device efficiency is ascribed to these phenomena. The importance of the combined use of various techniques on the same interface is stressed.

Electronic structural changes in multiwalled carbon nanotubes caused by adsorbates and Cs deposition were studied by photoemission spectroscopy. The adsorbates formed by air exposure slightly increased the work function. The increased work function is considered to be due to the surface dipole moment induced by the negatively charged adsorbates. On the other hand, the Cs deposition followed by the intercalation drastically decreased the work function. The results suggest that field emission current enhancement induced by adsorbates and by Cs deposition are based on essentially different mechanisms; that is, the former is associated with the formation of localized electronic states and the latter is mainly due to the work function decrease.

A common limitation in nanostructure research is often the requirement to perform experiments on ensembles of nanoparticles, therefore averaging over inherent distributions with respect to particle size and shape, chemical composition, crystallinity and defect structure. This limitation can be overcome by studying the properties of a single nanostructure individually, which will allow one to truly correlate scaling laws of material properties with changes in size. Here we report the first experiments to explore the feasibility of spectromicroscopy using a photoemission electron microscope (PEEM) to record the X-ray absorption spectra of single nanocrystals. Colloidal iron oxide nanocrystals with an average diameter and standard deviation of 13 nm and 2 nm, respectively, were deposited on graphite (HOPG) forming small islands of agglomerated γ-Fe₂O₃ nanocrystals (4–30 particles) as determined by scanning electron microscopy. Spatially resolved soft X-ray absorption spectra at the Fe L_3,2 edges of these individual islands were recorded with the PEEM2 instrument of the Advanced Light Source (ALS).

In near edge X-ray absorption fine structure (NEXAFS) spectra of carbon (C) and nitrogen (N) K-edge regions were measured for the stable radical compound lithium phthalocyanine (LiPc). The observed spectra were compared with those of metal free Pc (H₂Pc), copper Pc (CuPc), and zinc Pc (ZnPc). The spectral features in the C K-edge region of LiPc were different from those of other Pcs in the low photon energy region. An additional peak around 283.5 eV was observed only in the spectrum of LiPc. On the other hand, the spectral features of these MPcs in the N K-edge region were similar and did not show a corresponding extra peak. These results were explained by the patterns of molecular orbitals of these Pcs and the selection rules of X-ray absorption in the localized core-hole picture. The extra peak observed in the C K-edge region is assigned to the excitation from C1s to the highest occupied molecular orbital (HOMO), which is only singly occupied due to the radical nature of the phthalocyanine ring. Group-theoretical consideration indicated that the transition from N1s to the HOMO is not allowed, and this explains the lack of an extra peak at the N K-edge.

Absolute photoemission total yield (APTY) spectra and VUV-excited luminescence in cesium halides of CsCl, CsBr, and CsI, as well as in Tl-doped and Na-doped cesium iodide, were studied at several temperatures from 400 K to 80 K in the VUV region (5–30 eV). Although it is common that the intensities of APTY exceed unity in the photon energy region exciting Cs⁺5p core electron in all of these halides at room temperature, this is most in the cesium iodides and most less in CsCl. These phenomena are explained in terms of Auger enhancement due to the Auger effect associated with the decay process of the Cs⁺5p core hole. From the detailed analysis of temperature dependence of APTY and of Auger free luminescence (AFL) in CsCl and CsBr, a strong complementary relation between APTY and AFL was revealed, and the temperature-dependent energy level diagram of CsBr including the valence band and the Cs⁺5p core level was proposed. While in these CsI, APTY indicate as large as 1.5 or more at the high temperature and the decrease of APTY for decreasing temperature can be commonly explained in terms of the complementary relation with photoluminescence.

In order to investigate the structure of semiconductor/glass composites prepared by ion implantation, the grazing incidence small angle X-ray scattering (GISAXS) technique was applied to CdS nanocrystals synthesized in SiO₂ by implanting separately the constituent Cd and S atoms with a dose of 10¹⁷/cm² each, which resulted in a Gaussian depth density distribution of the dopants. Subsequently the samples were annealed at 700°C in order to form CdS particles. Due to the high concentration of nanocrystalline CdS, the scattered intensity is not following simple homogenous film models. Instead, additional particle scattering contribution is detected, as well as a multiplicative contribution due to inplane (lateral) particle correlation. From the first one, the particle size is estimated to be 4.6 nm, while an interparticle distance of 15–25 nm is deduced from the latter. Taking into account the applied dose, these values suggest that either part of the Cd and S ions are still dissolved in the amorphous substrate after annealing, and thus are not contributing to the CdS nanoparticle formation, or that implanted atoms have diffused deeper into the substrate during annealing.

We discuss a soft X-ray emission (SXE) valence band (VB) spectroscopy method for the study of the electronic structure and chemical phase composition of solids in a near-surface region with depth resolution. The depth information is obtained by variation of the energy of the incident electron beam used to excite the SXE spectra. As the information depth can be varied from about 1 nm to 1 μm in silicon, this method is suitable for the investigation of materials of modern micro- and nanoelectronics. VB → core level (Si 2p or Al 2p) transitions in Si-based materials are used to demonstrate the technique. It was found that the contribution of the signal from the near-surface region (< 1.5 nm) can be substantial (up to 50%) when the primary electron energy does not exceed the Si L_2,3 threshold by more than 150 eV. The technique is applied to Al impurities in a Si matrix, produced by ion implantation. The electronic structure at the Al sites and depth distribution of the Al impurity change markedly after postimplantation annealing. The observed electronic structure after annealing is in agreement with electronic structure calculations for substitutional Al impurities in a crystalline Si lattice.

The optical properties of a wide series of the tungstates with the scheelite and wolframite crystal structure at the threshold of the fundamental absorption region were studied. New information about the influence of the electronic states forming the bottom of the conduction band and the top of the valence band on the formation of emission centers and mechanisms of energy transfer to these centers was obtained.

The optical constants and dielectric functions of a series of samples of glassy carbon subjected to a heat treatment from 1000°C to 3000°C have been evaluated in the spectral region of 1.5–13.8 eV. In the region of 1.5–5.0 eV the method of spectroscopic ellipsometry and multiangle reflectivity with p-polarized light was applied, while in the VUV region the optical constants were evaluated by means of multiangle reflectivity with partially polarized light as well as by means of Kramers–Kronig analysis.

The optical spectra are discussed, in analogy with graphite, in terms of single-electron transitions and π electron plasma excitations. The changes in the spectra by heating are ascribed to increasing average size of ordered regions and sample densification in the absence of three-dimensional graphitization.

A complete polarization analysis of soft X-ray radiation with primary standards can be performed using a phase shifter in transmission and an analyzer in reflection. V/Ni multilayers produced at PSI by sputter deposition showed a phase shift |△| = 7.6° at 512 eV and a reflectance ratio R_s/R_p > 1000 for E in the range 507–512 eV. For Cr/C multilayers a phase shift |△| = 21° was found at 277 eV and R_s = 20%, R_s/R_p = 1400 at 278 eV. The values show that these components are suitable for polarization analysis at the mentioned energies.

For a spin-resolved inverse photoemission spectrometer, the most important component is the electron optics system consisting of a 90° deflector and lenses to transfer the spin-polarized electrons from a GaAs photocathode to the sample at high transmission. We adopt a cylindrical deflector when we construct a spin-resolved inverse photoemission spectrometer. A performance test shows that our electronic optics system has achieved 83% transmission, and also that the cylindrical deflector has no shortcoming compared to the spherical type.

At the Vavilov State Optical Institute (SOI) a special apparatus and a methodology have been developed which enable one to measure absolute fluxes from the full disk of the Sun in the VUV, the EUV and the soft X-ray spectral ranges (0.14–157 nm). The apparatus allows one to carry out absolute spectral measurements using simultaneous operation of the radiometer and spectrometers. The radiometer is equipped with a special disk on which several filters are fitted (foils, thin films and crystals). This enables one to carry out measurements in the spectral range of 0.14–157 nm. The EUV grating spectrometer with normal incidence provides five spectral channels with a channel width of 35 nm each for measurements in the spectral range under 155 nm. The spectral range of 1.8–60 nm can be covered by a soft X-ray grazing grating spectrometer. All instruments are equipped with the same detectors, namely with the open secondary electron multipliers (SEM) with photocathodes made of BeO which were developed at the SOI. This photocathode has a good sensitivity in the spectral range under 150 nm and its sensitivity falls practically down to zero at wavelengths greater than 160 nm. The SEM has a high amplification (up to 10⁸) and is stable to atmospheric influence. The stability of the SEM could be controlled by the radiation of the isotope ⁵⁵Fe (0.2 nm). There are plans to use this complex for mutual calibration of various synchrotron sources. Moreover it is planned to launch the Space Solar Patrol mission at the Russian Module of the International Space Station.

Scanning photoelectron microscopy (SPEM) was used to investigate a semiconductor sensor chip and Cu patterns embedded in Si substrate. It was found that the line shift in X-ray photoelectron spectroscopy (XPS) caused by local charges on the passivated surface is proportional to the distance between the surface and the conducting layer of the sensor chip. By using the line shift in XPS, inner-layered microstructure of the sensor chip could be imaged without destroying the sample. The result for the Cu patterns indicates that the surface of Cu patterns is more contaminated with carbon in the air than the surface of the Si pattern.

A spectrometer for soft X-ray emission spectroscopy was designed and constructed. The aims of this spectrometer are high detection efficiency and realizing the energy resolution of 1000 (E/ΔE) in a compact size. To satisfy these requirements, a flat field spectrometer with a varied line spacing grating and a CCD detector was chosen. Furthermore, the configuration without the entrance slit was employed. For compact flat field spectrometers, the spectral dispersion on the detector is sometimes smaller than the spatial resolution of existing detectors restricting maximum energy resolution. The spectral dispersion on the detector was optimized to balance with the spatial resolution of CCD. Optical performance was tested with a ray-tracing program. The spectrometer covers 250–900 eV, with two gratings. Taking into account the spatial resolution of a CCD detector, the maximum energy resolution of the spectrometer is estimated to be about 1000 (E/ΔE) for the beam size of 10 μm. Using the constructed spectrometer, test experiments were performed at the beamline BL27SU in SPring-8.

A new approach for investigations of circular dichroism in the angular distribution of photoelectrons (CDAD) is presented. The image contrast using a photoemission line of a certain material is combined with imaging of the angular distribution pattern using a photoemission electron microscope (PEEM). CDAD can be used to investigate pure scattering information by means of the same instrument in microscopically selected regions on a surface. The experiment combines angle-resolved XPS imaging with the indirect mapping of the local environment of atoms by means of CDAD holography. In a conventional photoelectron diffraction or photoelectron holography experiment, it is necessary to move the sample and/or the detector; for example, a rotatable electron analyzer is used to map a full angular pattern. In the present approach, the diffraction plane is mapped for a certain kinetic energy of electrons in the backfocal plane. An imaging high-pass energy filter has been used in combination with the PEEM in order to acquire surface images and/or diffraction pattern maps. Direct photoemission from the W-4f core levels was observed at different photon polarization to obtain two images and two diffraction patterns of the W-4f emission line using light of opposite helicity. The agreement of calculations with the experimental result is satisfactory. This constitutes a very encouraging starting point for the use of the CDAD holography together with the PEEM technique providing mesoscopic and atomic as well as chemical resolution in microselected areas on solid surfaces.

A time-resolving two-dimensional position-sensitive detector was applied to soft X-ray emission spectroscopy (SXES) and its feasibility was successfully studied by using synchrotron radiation. A soft X-ray emission spectrum synchronously acquired with bunch signal of the storage ring has elucidated that a threefold higher signal-to-noise ratio could at least be achieved. By adopting a pump–probe technique to SXES, we have preliminarily studied the effect of the electron–hole plasma (EHP) on the Si 2p soft X-ray emitting processes of Si crystal. A meaningful change in the excitation curve for the elastic scattering yields near the 2p thresholds was found to be probably due to the many-body interactions caused by the EHP.

A microscopic ultraviolet photoelectron spectrometer (micro-UPS) is developed using a Schwarzschild objective coated with the two-color multilayer which reflects efficiently both He-I (21.2 eV) and He-II (40.8 eV) resonance lines. The spot size by the use of a pinhole of 300 μm diameter is 5 μm, which was measured using the visible light from a Xe lamp. In the present micro-UPS, a pinhole of 2 mm diameter is used to gain sufficient photon intensity, so that the spot size is 25 μm. The valence band spectra of a NiWO₄ microcrystal, which cannot be grown in sufficient size, were measured by the present micro-UPS. The origin of atomic orbitals constituting the valence band of NiWO₄ was elucidated.

In order to study the electronic states of solids, using high-resolution, low-temperature photoemission spectroscopy (PES) with tunable photon energies, we have constructed a high-resolution undulator beamline operating in the photon-energy range 26–300 eV at the compact 700 MeV electron-storage ring HiSOR located at Hiroshima University. By way of gas-phase experiments, with photon flux larger than 2 × 10¹⁰ photons/s, photon-energy resolving powers of E/ΔE > 16000 and 3800 were obtained at hν ~ 48 eV and ~ 244 eV, respectively. High-resolution PES experiments on evaporated Au at 8 K were performed at the beamline with a total energy resolution of ΔE = 12 meV at hν = 48 eV. Among the first experiments from the beamline, a resonant photoemission study of UNiSn is presented. The U 5f derived peak is significantly enhanced at hν = 98 eV.

We report an experimental system dedicated for high-resolution and low-temperature photoemission spectroscopy installed on the helical-undulator beamline of a compact 700 MeV electron-storage ring (HiSOR) at Hiroshima University. A 3 m off-plane Eagle normal-incidence monochromator at the beamline covers the photon-energy range of hν = 4–40 eV. From the photoemission spectra near the Fermi edge of evaporated Au film, the total energy resolution was estimated to be 4.5 and 7.5 meV at hν = 7.1 and 22.6 eV, respectively. We have also performed photoemission measurements of Yb metal, which demonstrated highly bulk-sensitive spectra for the excitation by low energy photon. The high-resolution photoemission spectra of Y_{1 - x}Ca_xTiO₃ with x = 0.41 in the vicinity of the boundary between the metallic and insulating phases exhibited anomalous temperature dependence in comparison with those of normal metals.

A novel apparatus devoted to time-resolved photoemission experiments in the sub-picosecond regime will be presented. The system is composed of a Ti:sapphire laser source and a time of flight (TOF) electron energy analyzer mounted in a UHV experimental chamber. The laser source is characterized by a pulse duration of 150 fs at a wavelength of 790 nm (1.57 eV) and operates at a repetition rate of 1 kHz. To perform photoemission measurements, UV radiation up to 6.28 eV is produced with sequential steps of frequency conversion by employing crystals with a second order nonlinearity. Photoelectrons are collected by a TOF spectrometer designed to analyze electrons from tenths of eV up to 5 eV. It can be operated in two different angular resolution modes switching on and off an electrostatic collection optics: the high angular resolution mode (Δα = ±2.7°) and the low angular resolution mode (Δα = ±5.6°). Single photon photoemission spectra from the Ag(100) clean surface have been recorded at room temperature using the fourth harmonic light (λ = 200 nm and hν = 6.28 eV). The Fermi edge profile convoluted with a Gaussian-shaped energy transmission function of the TOF spectrometer sets an upper limit for the energy resolution which is about 65 meV (FWHM) at 2 eV of electron energy.

A new undulator beamline at the Advanced Light Source, Lawrence Berkeley National Laboratory is described. This new beamline has an Apple II type undulator which produces linearly and elliptically polarized X-rays. A high resolution monochromator directs the radiation to two branchlines. The first branchline is optimized for spectroscopy and accommodates multiple endstations simultaneously. The second branchline features a photoemission electron microscope. A novel feature of the beamline is the ability to produce linearly polarized radiation at arbitrary, user-selectable angles. Applications of the new beamline are also described.

Vacuum ultraviolet (VUV) and soft X-ray (SX) spectroscopic measurements are important means to diagnose radiation power loss, impurity ion densities and effective charge of confined plasma, Z_eff, in magnetically confined plasmas such as fusion plasmas. We have constructed space- and time-resolving flat-field VUV (150–1050Å) and SX (20–350Å) spectrographs by using aberration-corrected concave gratings with varied line spacing. Absolute calibration experiments have been conducted at the Photon Factory in the High Energy Accelerator Research Organization. Absolute sensitivities of the VUV and SX spectrographs have been obtained for the two (S and P) polarization geometries. Thus, absolute intensities of emission spectra from impurity ions can be measured together with their radial distributions in plasmas. The total radiation power was determined to be less than 6 kW within ±20% of error in our normal plasma operation. Density profiles of impurity ions were reduced by using absolute emissivities of impurity lines and a collisional-radiative model. Moreover, the value of Z_eff is estimated to be 1.00 in the tandem mirror GAMMA 10 plasma.

Superconducting tunnel junctions (STJs) are applicable to as photon detectors with an energy resolution and a high photon-counting rate. The absorption of a photon in the superconductor of the STJ generates a number of quasiparticles that is proportional to the photon energy. Especially for soft X-ray and extreme ultraviolet (EUV) applications, STJs have good performance because of their high absorption efficiency below 1 keV. We are developing an energy-dispersive photon detector for EUV radiation using STJs with Al trapping layers. We evaluated the performance of the detector for EUV photons by using the Synchrotron Facility at KEK-PF in Tsukuba, Japan. We achieved an energy resolution of FWHM=18 eV (including the external noise of 17.6 eV) for 55 eV EUV photons with a 100 × 100 μm² STJ. We present details of the junction design and discuss our experiments and the results.

The changes in phase contrast X-ray images as the photon energy is changed were observed and analyzed. Such results were justified in part with a preliminary model based on the interplay of refraction and absorption effects. The observed change may open up interesting opportunities for chemical analysis based on phase contrast images taken at different absorption edges.

Design, fabrication, and reflectance evaluation of multilayer mirrors for He-II radiation are described. Based on the available complex refractive indices of materials at the wavelength of 30.4 nm, Mg, Al, Si, Cr, and C were selected as small absorption material to be paired. Mo/Si and Sc found in previous works were also included. In referring to optimum design for the highest reflectance, multilayers of Al/Mg, Au/Mg, C/Mg, Sc/Mg, C/Si, Al/C, and Mo/Si were fabricated by ion-beam sputtering. The highest reflectance of 27% was observed by a 15-period Mg/Sc mirror.

An XUV beamline has been constructed on the SRS synchrotron radiation source at Daresbury Laboratory, exploiting the output from a 2 T multi-pole wiggler. The beamline is based on an SGM design and produces photons in the energy range 40–350 eV. By taking a 7.5 mrad fan, a high flux of between 1 × 10¹³ and 1 × 10¹⁴ photons/s/300 mA ring current can be delivered, with a resolving power of around 1000. A Kirkpatrick–Baez mirror pair focuses the light onto the entrance slit of the monochromator. Using 10 · m entrance and exit slits, a resolving power of up to 10,000 can be achieved, with a flux of 0.5–2 × 10¹¹ photons/s/300 mA. The calculated and measured performance is presented, with emphasis on the challenges associated with utilizing the XUV output of a high field MPW source. In addition, the scientific research programme is outlined.

A new angle-resolving electron energy analyzer composed of a conical electrostatic prism and a position-sensitive detector was developed for gas phase photoelectron spectroscopy. The performance of the analyzer has been tested by measuring photoelectron spectra of Ar using a helium discharge lamp. The angular resolution of 3° was achieved at the pass energy E of 5.6 eV. The best energy resolution was ΔE/E = 0.043 at E = 1.4 eV.

Multilayer polarizers for the use of He resonance lines have been developed. Si/Mg and SiC/Mg multilayers were designed and fabricated for the He-I and He-II resonance lines, respectively. The performance was checked by the use of synchrotron radiation. The polarizance and s-reflectance of the He-II polarizer measured at an angle of incidence of 40° were 0.98 and 41%, respectively. The polarizance of the He-I polarizer measured at an angle of incidence of 31.5° was 0.96.

The development of highly reflective multilayer mirrors for use in the wavelength region around 6 nm is desired for X-ray photoemission spectroscopy for inner-shell excitation using a Schwarzschild objective. For this application, reflectivity is the most critical parameter determining the performance of multilayer mirrors, because the reflectivity of multilayers in the 6 nm region is generally very low. We have designed CoCr/C multilayer mirrors with a comparatively high reflectivity at around normal incidence and have fabricated them by magnetron sputtering. The measured peak reflectivity is about 16% at a wavelength of around 6 nm and an incident angle of 88°. The reflectivity remains almost constant for 4 h under 300°C in an Ar atmosphere.

After having reached the status of user facilities in the infrared, Free Electron Laser (FEL) oscillators are becoming interesting light sources for scientific research also in the UV/VUV wavelength region. Thanks to continuous advances in accelerator and mirror technology, it has been possible to reach with tunability energies which are hardly accessible with conventional lasers (the European FEL project at Elettra recently lased below 190 nm), with the realistic perspective of reaching even shorter wavelengths in the near future. Thanks to their unique properties, such as tunability, full coherence, high intensity, spectral and temporal stability, they offer new opportunities for many kinds of spectroscopy: in particular, storage ring FELs are ideal sources for pump–probe experiments in conjunction with synchrotron radiation, as demonstrated by a series of studies of the nonequilibrium space charge distribution at photoexcited Si surfaces and interfaces performed at SuperACO. Besides, especially when operated at shorter wavelengths, FEL oscillators are very attractive also for one-photon experiments requiring high power and full tunability. We describe here the properties of the sources, and provide an overview of the experiments that have been performed and that are planned to exploit the new opportunities they offer.

Hot luminescence of self-trapped excitons in solid xenon has been investigated in a wide temperature range (from LHeT to 100 K), using X-ray and two-photon excimer laser (ArF and KrF) excitation. Hot luminescence of bulk Xe crystals of volume 2 cm³ appears to be very sensitive to both the temperature and the quality of the samples, while its highest intensity has been registered under KrF laser excitation. The experimental results are in good accordance with a recent nonperturbative quantum relaxation theory of strong local vibrational excitations.

Vacuum ultraviolet (VUV) continua emitted from a laser-produced plasma have been utilized for the spectroscopy of a solid state laser material. An absorption spectrum of 0.1%Nd:LaF₃ has been measured by use of the continuum emission. The minimum absorption coefficient of 1.4 cm^-1 was evaluated at 174 nm.

The spectroscopic properties of several fluoride crystals (LiYF₄, KYF₄, K₂YF₅, KLiYF₅, CsY₂F₇, SrF₂) doped with Nd³⁺, Er³⁺ or Tm³⁺ are analyzed from the viewpoint of their possible applications as active media for VUV solid state lasers. It has been found that in many crystals there exists very efficient nonradiative relaxation from higher-lying 5d states to the lowest 5d level responsible for the spin-forbidden luminescence. In fact, the energy level diagram of 5d–4f transitions for these two ions represents a typical four-level laser scheme with a considerably larger Stokes shift of 5d–4f luminescence from the edge of the strong spin-allowed 4f–5d absorption than for Nd³⁺, which causes smaller reabsorption of emitted VUV radiation in the crystals doped with Er³⁺ or Tm³⁺. Two crystals LiYF₄:Nd³⁺ and SrF₂:Er³⁺ were tested for laser action under pumping by a pulsed F₂ laser operating at 157 nm. However, no indications of the appearance of stimulated emission have been detected for both crystals. The possible reasons for the absence of lasing are discussed.

A XUV source that produces a strong emission band at the wavelength of 13.5 nm with a FWHM of 0.6 nm and a duration of about 100 ns is described. In particular this wavelength has attracted the attention of many scientists working in the field by being a good candidate for the development of XUV lithography. The source was generated by using an ablative capillary discharge where the capillary was made of PVC (polyvinyl chloride). A remarkable burst of radiation at the above wavelength was recorded, the intensity of the radiation being higher by a factor of 10 in the spectral region of interest, as compared to usually used capillaries made of POM (polyacetal), or to recently developed capillary discharges in noble gases. Total XUV radiation energy of up to 50 mJ per pulse seems to be possible with such a device. Due to its simplicity, the described capillary discharge is a good candidate for a simple incoherent XUV source at 13.5 nm.

We have performed the two-photon correlation measurement of synchrotron radiation in the VUV region to characterize the photon statistics of the light. The explicit bunching effect of the second-order coherence is observed, which implies that synchrotron radiation has a chaotic nature. We also estimate the electron beam emittance of a stored beam. The two-photon correlation measurement will be utilized to diagnose the storage ring and to characterize the coherence properties of X-ray free electron laser in the future.

An advanced experiment has been performed to improve X-ray laser output in the recombination plasma scheme. Two 11-mm-long plasmas were produced by laser irradiation so that each axis aligned into a line consisting the double-target configuration. X-ray intensity of the 15.47 nm transition line of the Li-like Al ion was enhanced in the double-target scheme. A half-cavity experiment in the double-target configuration was also performed, and the enhancement of X-ray was also observed. The results in the half-cavity experiment were consistent with an estimation, where the two plasmas having positive gain contribute to amplify X-rays passing through plasmas and reflecting from the multilayer mirror. Measurements have been started to clarify spatial coherence of the lasing X-ray beam by using the dispersing coherent diagnostic system.

The spatial profile of the transient collisional excitation Ni-like Ag X-ray laser in various plasma lengths was observed using the near field imaging method. The gain region was the size of 50 μm and 30–50 μm distant from the target surface. The shapes of the gain region were crescent shape or consisted of two spots.

A powerful source of broadband UV-VUV radiation is developed on the base of a multichannel sliding discharge. The discharge having a radiating surface of hundreds of cm² is initiated simultaneously along several dozens of parallel linear channels located close to each other. The design of the discharge electric circuit allows a remarkably fast energy deposition into plasma: up to 4 kJ during 1.5 μs. Absolute measurements of discharge VUV radiation are performed by the method of spectrally selective dynamic actinometry. The total photon flux produced by the source within the spectral range of 120–200 nm reaches 10²⁶ photons/s.

The differential spectra of bremsstrahlung on scattering of intermediate energy (0.3–2 keV) electrons by Ar atoms are studied. The measurements are made in the 6.5–10 nm ultrasoft X-ray region (the photon energy region 190.8–124 eV), where the dynamic polarizability of an Ar atom is low. For each of the wavelengths studied it is found that as the electron energy is increased from 0.3 to 0.7 keV, the bremsstrahlung intensity increases first in direct proportion to the square root of the scattered electron energy and then decreases in inverse proportion to it on subsequent rise in the energy from 0.8 to 2 keV. The increase in the bremsstrahlung intensity with electron energy is explained in terms of the low-energy approximation by the contributions of the opened channels of atom excitation and ionization that accompany the braking effect.

We describe a Fourier transform spectrometer designed to operate down to 60 nm on a synchrotron beam line. As far as we know, there is no such instrument available in the EUV (λ < 140 nm) partly because manufacturing accurate beam splitters remains the major difficulty at these wavelengths. We use a wave front division interferometer instead of an amplitude division one to overcome this difficulty. The interferometer is based on a modified Fresnel bimirror configuration, which is controlled by an original optical system. This system keeps the mirror tilt error to a negligible value during mirror translation, and provides a sensitive interferometric measurement of the mirror translation. The sampling interval is 29 nm (path difference), allowing one to record large band spectra down to λ = 58 nm with spectral resolution δσ = 0.33 cm^-1 for 512 K samples (one-sided interferograms).

We measured the apparatus function by recording an interferogram from a He–Ne stabilized laser. By studying the white noise in the corresponding spectrum, we found that the sampling error in the interferogram was about 0.4 nm rms, which produces a near-perfect apparatus function. Finally, we recorded the visible/near UV spectrum of an arc mercury lamp for illustration.