Advances in Multi-Photon Processes and Spectroscopy

The following sections are included:

• Preface

• Contents

State-selective coherent phase control using single shaped femtosecond laser pulses is implemented to achieve forward and backward shifts in time (up to a global arbitrary phase) of the rovibrational wave packet evolution Ψ (t) → Ψ (t − t_shift). Experimentally, the result is reflected as a time translation of the whole measured pump-probe transient. The wave packet is composed of eight rovibrational quantum states excited from a single selected rovibrational level (using a cw laser) of the electronic state. The high degree of control over the exact coherent superposition of the wave packet states is accomplished using a pulse shaping setup incorporating a liquid crystal spatial light modulator to encode the desired phases in the pump pulse. Beyond the direct implications of this work involving the experimental implementation of coherent control, it is also relevant to the possibility of probing molecular dynamics and coherent configurations that, without phase control, would occur only at long times after the wave packet excitation. This is relevant, for example, to highly dissipative environments where the wave packet dephases on short time scales.

No abstract received.

An application of a feedback control theory to two types of unimolecular reactions, an isomerization of hydrogen cyanide and an enantiomer preparation of phosphinothioic acid is presented. In controlling the isomerization, we treat the quantum feedback control theory in a classical way. An expression for the optimal laser pulse is derived from the classical Hamilton's equations of motion. The amplitudes of the optimized pulse at time t are proportional to the linear momentum of a representative point of the reaction. On the other hand, the linear momentum is constructed by the nuclear wave functions evaluated by solving the time-dependent Schrödinger equation. The kinetic energy of the representative point is the controlling parameter. The results of model calculation of the isomerization HCN → HNC in a two-dimensional model show that the present control method can design pulses for the isomerization with a high quantum yield. For quantum control of the racemic modification of phosphinothioic acid, the reaction coordinate is the torsional motion of the hydrogen atom around the P-S axis of the molecule. By using the quantum feedback control theory, we obtain the reaction product of ∼ 100% of a single enantiomer in the low temperature limits starting from the lowest initial state which is a coherent state representing two equivalent enantiomers with opposite chiralities. Mechanisms underlying the quantum control of the enantiomer preparation are discussed.

A variety of systems display power-law dephasing without a specific time scale at long times. Examples range from the classic spin-boson model at low temperature to highly laser-excited molecules undergoing vibrational dephasing. The specific mechanisms for this phenomenon as applied to molecular vibrations will be discussed, as well as general constraints that must be satified by the Hamiltonian in order to support slow decoherence. Experimental examples from the dephasing of the molecule SCCl₂ will be considered in detail. The data and calculations show anisotropic quantum diffusion of the vibrational wave packet in state space. The slow diffusion can be harnessed to control the vibrational motions of the molecule, leading to a 'static' coherent control scheme which achieves selective reactivity by combining coherent stabilization of vibrational wave packets with Franck-Condon control. This type of control scheme leads to a new criterion for degeneracy of interfering control paths: rather than ΔE=0, which is usually required to achieve product control in the asymptotic time limit, ΔE in ‘static’ coherent control is inversely proportional to the dissociation time scale. To address the experimental data, analytical calculations use perturbation expansions on the quantum state space lattice; time-dependent simulations are carried out with quantum symplectic propagators. The localization conditions and hierarchically scaled anharmonic couplings required for slow decoherence exist for most realistic coupled molecular, spin, or environmental systems: power law decoherence on intermediate time scales could rum out to be a very general phenomenon.

The role of four-wave mixing (FWM) techniques in coherent control is considered from the point of view of some of the most important developments in this field over the past years, namely multiphoton excitation, pump-dump methods, interference between coherent pulses, chirped laser pulses, and optimal control. FWM techniques provide a powerful platform for combining coherently multiple laser pulses. We explore the effectiveness of these techniques in controlling chemical reactions. The phase relationship between the pulses is maintained by detecting the signal in a phase-matching direction. The results presented show control over the observed dynamics from ground and excited state populations. The FWM signal results from the polarization of the sample following three different electric field interactions. The virtual echo sequence is achieved by the interactions of the sample with three consecutive electric fields characterized by exp[i(kx-ωt)], exp[-i(kx-ωt)] and exp[i(kx-ωt)]. This sequence allows control over the observed ground or excited state dynamics. With the photon echo pulse sequence, characterized by interactions with exp[-i(kx-ωt)], exp[i(kx-ωt)], and exp[i(kx-ωt], we find that control of ground and excited state populations is not achieved. Differences between these two pulse sequences are shown experimentally and illustrated using wave packet simulations. Data obtained using the ‘mode suppression’ technique, in which the timing between the first and third laser pulses is fixed while the second pulse is scanned are presented. We show that this technique does not suppress the observed vibrational coherence from the ground or excited state but it yields an additional component to the signal that is independent of the vibrational coherence of the sample. Spectrally dispersed FWM is shown to be an ideal tool for studying intramolecular dynamics and this idea is applied to understanding the role of chirp in controlling molecule-laser interactions. All coherent control methods are affected by the rate of decoherence of the sample. Here we show how these rates are measured with FWM techniques. The measurements presented here illustrate how photon echo measurements yield the homogeneous relaxation rate while the virtual echo measurements yield the sum between homogeneous and inhomogeneous relaxation rates.

We have developed the “optical collision (OC)” approach to reach and control the transition regions of atomic and molecular interactions. The approach is based on the photoabsorption by a collisional quasimolecule during a single binary collision. Here I present two different OC approaches; one is the time-domain approach based on the fs pump-probe technique, and the other is the frequency-domain approach based on what we call the “second-order optical collision (SOOC)”. The time-domain approach has been applied to the Hg−CO van der Waals (vdW) complex, which is regarded as the bound quasimolecule, to realize the first observation of wave packet motion associated with an intermolecular energy flow in the vdW interaction. Possibilities of chirped pump-pulse and double pump-pulse control of the relevant energy flow are discussed. For the frequency-domain approach, we have applied the “wing-wing double resonance” technique to the Hg−Ar collisional quasimolecule to observe an ultrafast SOOC, where two-color, two-photon absorption takes place during a single binary collision, with a pair of nanosecond laser pulses. This experiment serves as the optical switching of an ultrafast thermal atomic collision; a collision pathway is controlled by changing the frequencies of those laser pulses. It is also shown that an ultrafast pump/probe measurement can be carried out with sub-picosecond time resolution using nanosecond pulsed lasers.

In coherent control experiments the product signal intensity is modulated by interference between two excitation paths. This modulation is produced by varying the relative phase of the electromagnetic fields used to excited the target. It is observed that the modulated signals for different channels may be out of phase with respect to each other. The phase lag between different channels is energy dependent and contains information about the dynamics of the system. This paper explores different mechanisms that produce such phase lags and assesses what may be learned from them.

We propose a new method for the direct determination of the quantum phase of continua utilizing the phase of linearly/circularly polarized lasers. With this method the phase difference of continua with opposite as well as same parities can be directly determined from the phase lag of the angle-resolved photoelectron signal with respect to the relative phase of lasers. For illustration, the proposed method is specifically applied to the Na atom to determine the phase differences between s, p, and d waves.

Fano interference arising from e.g. tunnel coupling of two semiconductor quantum wells can yield vanishing absorption of incident resonant radiation with nonzero stimulated emission. This is the basis for Harris-Fano lasing without inversion. It is here shown that it is interesting to analyze the problem in a basis such that the Fano decay matrix is diagonal.

We show that a strong nonresonant nanosecond laser pulse can be used to align neutral molecules. Our technique, applicable to nonpolar as well as polar molecules, relies on the anisotropic interaction between the strong laser field and the induced dipole moment of the molecules. The degree of alignment is enhanced by increasing the laser intensity or by lowering the initial rotational temperature of the molecules. We measure the alignment by photodissociating the molecules with a femtosecond laser pulse and detecting the instantaneous direction of the photofragments by ion imaging techniques. The highest degree of alignment is ≪cos²θ≫ = 0.81.

We have demonstrated alignment of neutral molecules and complete bleaching of electronically excited states by strong IR laser pulse. The electric field of a pulsed laser is used for alignment that is demonstrated by measurement of the control of the anisotropy of photofragments generated by polarized light. The angular distribution of I(²P_3/2) atoms from the photodissociation of CH₃I at 304 nm was measured with the 1.06 um laser pulse excitation:

We theoretically investigate the electronic dynamics and structure of and a one-dimensional H₂ model in intense laser fields. The time-dependent Schrödinger equations for the systems are exactly solved by the dual transformation method developed by the authors. We reveal the dynamics of bound electrons and the subsequent ionization process in terms of “field-following” adiabatic states. The mechanisms of enhanced ionization in the one- and two-electron molecules are examined. For the H₂, ionic states work as a main doorway state to ionization. We also show the simple characteristic feature of molecular electronic structure in an intense field: the electronic state of each atom in the molecule can be specified in terms of adiabatic states of the atom (or its anion) in the field. Control schemes leading to electron transfer and formation of new bonds are proposed.

The Coulomb explosion processes of H₂O, i.e., (i) H₂O³⁺ → H⁺ + O⁺ + H⁺ and (ii) H₂O⁴⁺ → H⁺ + O⁺ + H⁺, in ultrashort intense laser fields (∼ 1 PW/cm²) are investigated by the mass-resolved momentum imaging (MRMI) technique. From the analysis of these MRMI maps of the atomic fragment ions, it is found that the ∠H-O-H bond angle of the parent ions exhibits a significantly broad distribution (FWHM ∼ 60°) centered at the linear configuration and the O-H bond lengths for H₂O³⁺ and H₂O⁴⁺ become 1.7 and 2.0 times as long as R_e = 0.958 Å, respectively. The characteristic ultrafast bond angle deformation occurring prior to the Coulomb explosion is interpreted by the formation of the light-dressed potential energy surfaces of H₂O⁺.

We examine strong field atomic physics in a wavelength region (3-4 μm) where very little work has previously been done. The soft photon energy allows the exploration of one- electron atoms with low binding energies (alkali metals). We find that photoionization spectra differ from rare gas studies at shorter wavelengths due to more complex ion core potentials. Harmonic generation is studied, and we find that harmonic bandwidths are consistent with theory and the possibility of compression to pulse widths much shorter than that of the driving pulse. Harmonic yields in the visible and UV are sufficient for a complete study of their amplitude and phase characteristics.

Two new ways of controlling molecular processes are proposed. One is to sweep laser frequency and/or intensity at avoided crossings among dressed states to control nonadiabatic transitions there. The second is to use the intriguing phenomenon of complete reflection in the nonadiabatic tunneling type transition in the time-independent framework. The newly completed semiclassical theory of nonadiabatic transitions can give a nice analytical formulation for these.

We explore vibrational coherences in condensed phase molecules which are created by simple phase modulated femtosecond laser pulses. Experiments are performed using optical heterodyne detected Raman-induced Kerr effect spectroscopy, which independently detects the real (birefringence) and imaginary (dichroism) components of the third order molecular response. For an excitation pulse with a specific duration and phase modulation, we find the experimental signals arising from the transient birefringence and dichroism to be qualitatively different. The experimental results are interpreted using a wavepacket picture.

We observed coherent molecular vibration coupled to the Frenkel exciton in J-aggregates. The coherent oscillation in the induced absorbance change is explained by the modulation of the transition dipole moment through the 244cm⁻¹ -ruffling mode.

Ultrafast dynamics of excitations in polydiacetylenes was studied with the shortest visible pulses with sub-5-fs duration. With this high time resolution it was clearly found that the relaxation from 1¹B_u free-excitons to 2¹A_g self-trapped excitons takes place with the time constant of about 70fs. The frequencies of C=C (23fs) and C−C (27fs) stretching modes are modulated with 145fs-period(C−C=C bending).

This lecture reviews recent theoretical work on the role of continuous-wave radiation in extending the lifetime of localized states in dissipative two-state systems. Through a combination of path integral methods and semiclassical arguments it is shown that strong fields can stabilize localized states over long time intervals. At moderate dissipation strength this phenomenon is quite robust and insensitive to details of the phonon spectrum, temperature and driving frequency, depending primarily on the intensity of the applied field.

Wave packet motion is observed to be dependent on chirp direction and rate of excitation by femtosecond pulses in a cyanine dye molecule and a halogen-bridged metal complex. Strong reduction in excited state population is efficient for negatively chirped pulses in the cyanine dye molecule, which is explained in terms of a pump-dump process. Vibrational oscillation for positively chirped excitation is retarded by π/4 with respect to negatively chirped excitation in the halogen-bridged metal complex, which originates from the difference between excited and ground state wave packets.

Electromagnetic and one structural coherent control schemes for semiconductors are discussed. These elementary schemes manipulate electronic inter(sub)band transitions and allow the coherent control of a variety of physical phenomena and processes, such as photo–absorption and emission, phonon emission, excitonic (many–body) effects, and THz emission. Here, we specifically address the use of sub-picosecond two–color pulses to control photo–absorption in bulk semiconductors and semiconductor quantum wells, as well as the initiation of coherent charge oscillations in double wells. It is shown that the phase sensitivity in absorption arises from the presence of the electron–electron interaction. It can be maintained as long as the pump pulse duration does not significantly exceed the inverse beating frequency associated with the two–color pump pulse.

We have constructed a conventional far infrared spectrometer based on terahertz (THz) radiation as a spectral source. The THz radiation is generated by focusing a femtose cond pulse onto a semiconductor substrate placed in a strong magnetic field. Far infrared spectra of various condensed phase systems including neat liquids, supercritical fluids, a charge-transfer complex solution, and a protein solution have been measured by this spectrometer.