Latest Advances in Atomic Cluster Collisions

The following sections are included:

• PREFACE

• LOCAL ORGANISING COMMITTEE

• CONFERENCE PHOTO

• CONTENTS

Relatively long half-lives of the new nuclides open possibilities for the study of the structure of superheavy atoms, in particular chemical properties of the new elements. Experiments on the observation of the socalled “relativistic effect” in the electron structure of elements 112 and 114 using express radiochemical methods are now underway at the ⁴⁸Ca ion beam. The search for the most long-lived superheavy elements in nature is aimed at the registration of decay of element 108 isotopes. Preliminary results of measurements of spontaneous fission rare events in the underground laboratory at Modane (France) are discussed in context of the maximal nuclear half-lives at the top of the island of stability and in context of setting up new experiments.

An overview of present experimental investigation of superheavy elements is given. Using cold fusion reactions which are based on lead and bismuth targets, relatively neutron-deficient isotopes of the elements from 107 to 113 were synthesized at GSI in Darmstadt, Germany, and/or at RIKEN in Wako, Japan. In hot fusion reactions of ⁴⁸Ca projectiles with actinide targets, more neutron-rich isotopes of the elements from 112 to 116 and even 118 were produced at FLNR in Dubna, Russia. Recently, part of these data, which represent the first identification of nuclei located on the predicted island of superheavy elements, were confirmed in two independent experiments. The data are compared with theoretical descriptions.

Strongly coupled processes of deep inelastic scattering, quasi-fission and fusion are described concurrently with a common set of variables within the Langevin-type equations of motion. Shell effects on the multi-dimensional potential energy surface play an extremely important role in these reactions. This leads to the two-body (shape isomeric states in fission) and three-body clustering phenomena in heavy nuclear systems. Enhanced yield of the nuclides far from the projectile and target masses was found in the multi-nucleon transfer reactions due to shell effects. This suggests that the low-energy damped collisions of transactinide nuclei may be used as an alternative way for the production of surviving superheavy long-living neutron-rich nuclei.

A nuclear molecule or a dinuclear system consists of two touching nuclei which carry out motion in the internuclear distance and exchange nucleons by transfer. The dinuclear system model can be applied to nuclear structure, fusion reactions leading to superheavy nuclei, multi-nucleon transfer and fission.

The properties of nuclei embedded in an electron gas are studied within the relativistic mean-field approach. These studies are relevant for nuclear properties in astrophysical environments such as neutron-star crusts and supernova explosions. The electron gas is treated as a constant background in the Wigner–Seitz cell approximation. We investigate the stability of nuclei with respect to α and β decay. We find that the presence of the electrons leads to stabilizing effects for α decay at high electron densities. Furthermore, the screening effect shifts the proton drip-line to more proton-rich nuclei, and the stability line with respect to β decay is shifted to more neutron-rich nuclei. Implications for the creation and survival of very heavy nuclear systems are discussed.

One of the challenges posed by the demand for clean urban transportation is the compact and cyclically recoverable storage of energy in quantities sufficient for propulsion. Promising routes, such as the reversible insertion of Li⁺ ions inside solids for ‘rocking chair’ batteries, require a deformable host material with no irreversibility. Such ‘soft’ deformations are in general highly complex, but the compressibility of atoms or larger systems can be studied directly in situations with simpler symmetry. Thus, the search for ‘soft’ materials leads one to consider certain types of cluster, as well as linear or nearly-spherical structures (chains of metallofullerenes, for example) whose deformations can be computed from the Schrodinger equation. Extended or ‘giant’ atomic models allow one to construct compression-dilation cycles analogous in a rough sense to the Carnot cycle of classical thermodynamics. This simplified approach suggests that, even for idealised systems, there are constraints on the reversible storage and recovery of energy, and that (when applied to realistic structures) modelling based on such principles might help in the selection of appropriate materials.

A scheme we have formulated recently for partitioning the total dipole moments and polarizabilities of finite systems into site-specific contributions is used to analyze the structure-/shape- and size-specific aspects of the dipole moments and polarizabilities of small sodium clusters. The procedure is based on dividing the system volume into cells associated with its atoms. The site-specific, or atomic, dipole moments and polarizabilities are computed from the charge densities within the individualcells (“atomic volumes”) and the changes in these densities in response to an external electric field. The atomic dipole moments and polarizabilities are further partitioned into local (or “dipole”) and “charge-transfer” components. It is shown that the polarizabilities associated with the individual Na atoms vary considerably with the structure/shape of the cluster and the location of the atom within a given structure. Surface atoms, especially those at edges, have larger polarizabilities than interior atoms. The contribution of the charge-transfer components to the total polarizability increases with the cluster size.

The optimized structure, electronic and magnetic properties of La clusters consisting of up to 14 atoms have been investigated using ab initio theoretical methods based on density-functional theory. We show that increase in cluster symmetry can promote ferromagnetic instability in La clusters. A giant enhancement of magnetism in La₄, La₆ and La₁₃ clusters is reported. We also find that the ground states of La₂, La₃, La₅, La₇, La₉–La₁₁, and La₁₄ clusters possess nonzero magnetic moments that ranged from ∼0.1–1.0 μ_B per atom. Strong dependence of the magnetic moment on temperature for T > 300 K is predicted. The results obtained are compared with the available experimental data and theresults of other theoretical works.

The optimized structure and electronic properties of neutral, singly and doubly charged strontium clusters have been investigated using ab initio theoretical methods based on density-functional theory. We have systematically calculated the optimized geometries of neutral, singly and doubly charged strontium clusters consisting of up to 14 atoms, average bonding distances, electronic shell closures, binding energies per atom, and spectra of the density of electronic states (DOS). It is demonstrated that the size-evolution of structural and electronic properties of strontium clusters is governed by an interplay of the electronic and geometry shell closures. The influence of the electronic shell effects on structural rearrangements can lead to violation of the icosahedral growth motif of strontium clusters. It is shown that the excessive charge essentially affects the optimized geometry of strontium clusters. Ionization of small strontium clusters results in the alteration of the magic numbers. The strong dependence of the DOS spectra on details of ionic structure allows one to perform a reliable geometry identification of strontium clusters.

We present a new single-particle shell model, derived by solving the Schrödinger equation for a semi-spheroidal potential well. It may be used to study atomic clusters deposited on a planar surface. By assuming opacity of the surface only the negative parity states of the Z(z) component of the wave function are allowed, so that new magic numbers are obtained. The maximum degeneracy is reached at a superdeformed semi-spheroidal prolate shape whose magic numbers are identical to those obtained at the spherical shape of the spheroidal harmonic oscillator.

We report on electric deflection experiments of di-substituted benzene derivative molecules [para, meta and ortho amino-benzonitrile (PABN, MABN and OABN) and para and meta dimethyl-amino-benzonitrile (PDMABN and MDMABN), and para-fluoroaniline (PFAN)]. They are used as prototypes to study the influence of the asymmetry and rotation–vibration couplings in deflection experiments. Experimental deflection profiles are compared to results of ab initio calculations in the framework of the rigid rotor Stark effect and of the statistical linear response.

We have analyzed the stability and fission dynamics of singly and multiply charged neon cluster ions. The critical sizes for the observation of long-lived ions are for charge states 2 and 3 with a measured n₂ = 284 and n₃ = 656, respectively, a factor 3 to 4 below the predictions of a previously successful liquid-drop model. The preferred fragment ions of fission reactions are surprisingly small and their kinetic energy distributions peak at 200 meV or below. The size of these fragments and their average kinetic energies are much less than those predicted by the liquid-drop model. In contrast, magic numbers observed here for singly charged cluster ions are in accordance with previous observations for the otherrare gases.

This is an investigation of the dynamical screening of an atom confined within a fullerene of finite width. The two surfaces of the fullerene lead to the presence of two surface plasmon eigenmodes. In the vicinity of these two frequencies, there is a large enhancement of the confined atom's photoabsorption rate. The dynamical screening factor is given for two cases. In the first case, the atom is fixed at the center of the fullerene. The effect of the interaction between the dipole moments of the shell and the atom on the dynamical screening factor is investigated. In the second case, it is calculated for the atom placed at an arbitrary position inside the fullerene. A method for obtaining the spatially averaged screening factor is outlined.

Mass selected fullerene ions are exposed to synchrotron radiation in the 17–300 eV energy range. Selected absolute cross-sections for single and multiple ionization as well as fragmentation were measured for ions of C₆₀, C₇₀, C₈₀, C₈₂, and C₈₄. More recently, the first ever experiments with endohedral and fullerene ions have been conducted.

Absolute cross-sections for dehydrogenation of an ethylene molecule onto , Ti_nO⁺ (n = 2−25), , and were measured as a function of the cluster size, n, in a gas-beam geometry at a collision energy of 0.4 eV in an apparatus equipped with a tandem-type mass spectrometer. It is found that (i) the di-dehydrogenation proceeds on and Ti_nO⁺, and the mono-dehydrogenation on , and , (ii) the di- dehydrogenation cross-section increases gradually with the cluster size of and Ti_nO⁺, (iii)the mono-dehydrogenation cross-section increases rapidly above a cluster size of ∼ 18 for , ∼13 and ∼ 18 for , and ∼ 10 for , and (iv) therapid increase of the cross-section for (M = Fe, Co and Ni) occurs ata cluster size where the 3d-electrons start to contribute to the highest occupied levels of . These findings lead us to conclude that the 3d-electrons of lay a central role in the dehydrogenation on .

Gas-phase synthesized vanadium–mesitylene 1:2 V(mes)₂ cluster cations were size-selectively deposited onto a long-chain alkanethiolate self-assembled monolayer-coated gold substrate under ultrahigh vacuum conditions. Examination of the resultant clusters was conducted by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD), showing the clusters molecularly adsorbed and maintaining a sandwich structure on the substrate.

An atomic scale model has been developed to study the response of the pure nickel material during nanoindentation with shallow indentation depths (necessarily used for thin film applications) as an alternative to the frequently used continuum methods.

Quantum variational and diffusion Monte Carlo (VMC, DMC) calculations are carried out using accurate, ab initio interaction potentials for alkali metal ions doping small ⁴He clusters. The results for Li⁺, Na⁺ and K⁺ with clusters up to n = 16 reveal an interesting interplay between ionic forces and He−He interactions when driving the spatial delocalization of the solvating helium adatoms which form the initial snowball structures within the larger droplets.

We suggest a theoretical method based on statistical mechanics for treating the α-helix ↔ random coil transition in polypeptides. This process is considered to be a first-order-like phase transition. The developed theory is free of model parameters and is based solely on fundamental physical principles. We apply the developed formalism to the description of thermodynamical properties of alanine polypeptides of different lengths. We analyze the essential thermodynamical properties of the system, such as heat capacity, phase transition temperature, and latent heat of the phase transition. Also, we obtain the same thermodynamical characteristics from molecular dynamics simulations and compare the results with those of statistical mechanics calculations. The comparison proves the validity of the statistical mechanics approach and establishes its accuracy.

The evaporative cascade of neutral atoms from clusters is investigated using phase space theory and kinetic Monte Carlo simulations. We focus here on the kinematics of sequential dissociation, by recording the translational kinetic energy distributions of the fragments for thermal excitations at fixed excitation energy or at fixed canonical temperature. The average kinetic energy of the monomers exhibits non-monotonic variations with excess energy, backbendings being found as the consequence of successive cooling processes. These effects are significantly washed out by the thermal broadening of the initial excitation. However, these backbendings do not necessarily reflect the intrinsic caloric curves of the clusters.

We present a recently introduced hierarchical model for the description of clusters in contact with an environment (embedded or deposited cluster). We briefly outline the ingredients of the model and show the relevance of a proper treatment of the degrees of freedom of the environment. We then discuss the effects of cluster–substrate interaction in three different dynamical scenarios: optical response of a small Na cluster deposited on an MgO surface, a comparison of the dynamics of deposition of an Na cluster on an MgO surface versus an Ar surface, and strong laser excitation of an Na cluster embedded in an Ar matrix.

We present an interferometric pulse shaper set up for unrestricted phase, amplitude, and polarization pulse control. It is realized by integrating a 4f-shaper set up in both arms of a Mach–Zehnder interferometer and rotating the polarization by 90° in one arm before overlaying the phase and amplitude modulated beams. We demonstrate the capabilities of this set-up by introducing a method for generating parametrically tailored three-dimensional electrical fields of femtosecond laser pulses.

Structure, stability and properties of a new type of cluster, electron-positron quantum cluster, are discussed. The analysis is based on both the non-relativistic Hartree–Fock and self-consistent local-density approximations. An essential role of many-body effects in the formation of the droplets is demonstrated. Their properties are compared with the known physical objects such as metal clusters and clusters of excitons in a solid.

In this contribution, we discuss aspects of spectral tuning of molecular chromophores at the atomic level. The basis for the discussion is a series of measurements performed at the electrostatic ion-storage ring ELISA at the University of Aarhus. The experimental technique, which is a type of “action spectroscopy” based on photo dissociation, will briefly be discussed. We will present examples of spectral tuning due to external charges and proton exchange with the hosting medium.

Coarse-graining the potential energy surface in terms of its local minima, and the transition states that connect them, provides a framework for global optimization, and the calculation of global thermodynamic and kinetic properties. Here, we provide an overview of the computational tools used in such analyses, and present some new results for kinetic analysis of pathways obtained from a stationary point database for the GB1 hairpin peptide.

A theoretical framework for the prediction of nuclear magnetic resonance (NMR) residual dipolar couplings (RDCs) in unfolded proteins under weakly aligning conditions is presented. The unfolded polypeptide chain is modeled as a random flight chain while the alignment medium is represented by a set of regularly arranged obstacles. For the case of bicelles oriented perpendicular to the magnetic field, a closed-form analytical result is derived. With the analytical expression obtained, the RDCs are readily accessible for any locus along the chain, for chains of differing length, and for varying bicelle concentrations. The two general features predicted by the model are (i) RDCs in the center segments of a polypeptide chain are larger than RDCs in the end segments, resulting in a bell-shaped sequential distribution of RDCs, and (ii) couplings are larger for shorter chains than for longer chains at a given bicelle concentration. The presented framework is an important step towards a solid theoretical foundation for the analysis of experimentally measured RDCs in unfolded proteins in the case of alignment media such as polyacrylamide gels and neutral bicelle systems which align biomacromolecules by a steric mechanism. Various improvements and generalizations are possible within the suggested approach.

The unbinding process of the mAb4–4–20:fluorescein complex was investigated by means of a computational approach at the atomistic classical mechanical level, probing only a relevant set of generalized coordinates in order to determine the putative dissociation paths of the system. The complex problem was reduced to a low-dimensional scanning along a selected distance between the protein and the escaping ligand. The unbinding was further characterized by assessing the relative positional and orientational coordinates of the ligand. Solvent effects were accounted for by means of the Poisson–Boltzmann continuum model. The complex's dissociation time was derived from the calculated barrier height, in compliance with the experimentally reported Arrhenius-like behavior. The computed results are in good agreement with the available experimental data.

In an earlier work, it was suggested that the “magnetic sense” in birds may be mediated by the blue light receptor protein, cryptochrome, which is known to be localized in the retinas of migratory birds. The present contribution discusses an alternative mechanism of avian magnetoreception based on the interaction of two iron minerals (magnetite and maghemite), recently found in subcellular compartments within the sensory dendrites of the upper beak of several bird species. The analysis of forces acting between the iron particles shows that the orientation of the external geomagnetic field can significantly change the probability of the mechanosensitive ion channels opening and closing. Our theoretical analysis shows that the suggested magnetoreceptor system might be a sensitive biological magnetometer providing an essential part of the magnetic map for navigation.

The investigation of fragment length distributions of plasmid DNA following ion irradiation leads to a better understanding of the induction of DNA damage, particularly the clustering of double strand breaks. We present a model that calculates the fragment distributions of plasmid DNA following heavy ion irradiation by combining the Local Effect Model with a statistical model initially developed for X-rays. The integration of experimental constraints into the model calculations changes the resulting distributions strongly. We find a good agreement of our simulations with experimental fragment distributions based on atomic force microscopy studies. The model provides the means to rapidly predict the results for all ions. It may thus help to find the best ion species in order to demonstrate the impact of the localized energy distribution of particles.

We study the propagation of carbon nuclei in tissue-like media within a Monte Carlo Model for Heavy-Ion Therapy (MCHIT) based on the GEANT4 toolkit. The results of the calculations are compared with experimental data on depth–dose distributions and fragment fluences obtained with water phantoms irradiated by ¹²C beams. The MCHIT model describes well the depth–dose distributions and the attenuation of the primary beam due to nuclear fragmentation reactions, as well as the production of boron, beryllium and lithium fragments. However, the model should be improved with respect to the description of helium and hydrogen yields at different depths. We also made predictions for the doses at large distances from the beam axis, which are expected mostly from secondary neutrons, protons and alpha particles. The MCHIT capability of calculating three-dimensional dose distributions is demonstrated. The distributions obtained separately for each kind of secondary fragments provide information for calculating biological dose distributions, as the biological effects of ions depend on their charge, mass and energy.

We discuss different physical processes which may be responsible for biological damage induced by an ion beam propogating through living tissue. The creation of free radicals and an electron plasma, dissociative attachment of low-energy electrons, and local heating mechanisms are considered. Numerical estimates are performed for the case of carbon ion beams. We conclude that all three mechanisms are capable of inflicting single and double strand breaks in DNA molecules and thus can be responsible for the observed enhanced biological effectiveness of ion beams in cancer therapy.

We review some recent results of modeling the pattern formation by segmentation genes during the early development of the Drosophila embryo. The study of gene expression patterns is based on the “gene circuit” method consisting of four steps: obtaining gene expression experimental data, formulating a model, fitting the model to the data, and inferring new biology from the analysis of results. The biological data has the form of processed images of immunostained embryos and is adopted in the form of concentration curves for proteins coded by various segmentation genes averaged over many embryos. The model is formulated as deterministic reaction-diffusion equations with protein concentrations in many cell nuclei as state variables. The values of parameters in the model are calculated by fitting the solution of model equations to the experimental concentration curves. We also describe how the gene circuit approach allows one to elucidate a role in the pattern formation played by nuclear cleavages in the developing embryo.

The following sections are included:

• AUTHOR INDEX