É. T. Verkhovtseva

Cluster size effects in vuv radiation spectra of argon and krypton supersonic jets

Abstract The relation between VUV radiation spectra of argon and krypton supersonic jets excited by an electron beam and the mean size of clusters in a jet is studied. For each atomic and molecular emission of argon in the 104–120 nm range and of krypton from 116 to 130 nm, the maximum intensity is found to occur when electrons collide with clusters of a definite mean size. It is found that self-localized excitons of atomic type or diatomic excimer molecular type with a small binding energy (several hundredths of an eV) are mainly formed in outer icosahedral shells of argon or krypton clusters and those of diatomic excimer molecular type with a large binding energy (several tenths of an eV) arise principally in inner shells.

New emission continua of rare-gas clusters in the VUV region

Abstract New VUV continua of argon, krypton and xenon clusters excited by an electron beam are studied experimetally and theoretically. These continua are found to be due to the radiative decay of excimer molecules R * 2 (R is the rare-gas atom) formed in ionic argon, krypton and xenon clusters.

Size effect in the desorption of excited atoms and molecules from clusters of inert elements under electron bombardment

New channels of desorption of excited atoms and molecules from clusters of argon, krypton, and xenon under electron bombardment are found by the method of vacuum ultraviolet emission spectroscopy. The maximum yield of particles is registered in the interval of average sizes of 50–100 atoms/cluster. The regularities and features of the desorption of excited particles are revealed, making it possible to establish a new desorption mechanism called nonradiative excimeric dissociation. It is shown that the main stages of this mechanism are: the formation of molecular centers of the nature of highly excited diatomic excimer molecules in clusters in the process of self-trapping of high-energy p excitons (n=1); the nonradiative dissociation of these molecules to excited atoms and atoms in the ground state with large kinetic energies. It is established that the appearance of new channels of desorption of excited atoms and molecules from clusters under bombardment by electrons is due to features of the physical pro...

Atomic dynamics and the problem of the structural stability of free clusters of solidified inert gases

The dependence of the rms amplitudes of atoms in free clusters of solidified inert gases on the cluster size is investigated theoretically and experimentally. Free clusters are produced by homogeneous nucleation in an adiabatically expanding supersonic stream. Electron diffraction is used to measure the rms amplitudes of the atoms; the Jacobi-matrix method is used for theoretical calculations. A series of distinguishing features of the atomic dynamics of microclusters was found. This was necessary to determine the character of the formation and the stability conditions of the crystal structure. It wass shown that for clusters consisting of less than N∼103 atoms, as the cluster size decreases, the rms amplitudes grow much more rapidly than expected from the increase in the specific contribution of the surface. It is also established that an fcc structure of a free cluster, as a rule, contains twinning defects (nuclei of an hcp phase). One reason for the appearance of such defects is the so-called vertex in...

THE EFFECT OF NON-EQUILIBRIUM GAS CONDENSATION IN VUV SPECTRA OF ELECTRON-EXCITED SUPERSONIC ARGON JET

The non-equilibrium condensation of argon in a supersonic jet is studied by mass spectrometry, electron diffraction and VUV emission spectroscopy. The peculiarities of the dependence of atomic and molecular VUV emission intensities on the gas pressure and temperature at a nozzle entry is shown to be due to changes in the jet phase composition and structure during the gas condensation.

Detection of stacking faults in carbon dioxide clusters

An electron-diffraction study of clusters formed in supersonic jets of carbon dioxide is reported. The mean size of the clusters varies from 4×102 to 2×104 molecules/cluster. From an analysis of the diffraction patterns obtained it is established that clusters of these sizes have the structure of the bulk crystal. Stacking faults are observed for the first time in clusters with an average size of less than 1.7×103 molecules/cluster; the density of these stacking faults increases with decreasing size of the aggregations. It is found that the density of stacking faults in (CO2)N clusters is considerably lower than rare-gas clusters.

On the mechanism of transformation of icosahedral rare-gas clusters into fcc aggregations

The experimental diffraction patterns from Kr cluster beams are compared with the diffraction functions calculated for rare-gas clusters with dimensions of 3×103 and 1×104 atoms/cluster. The experimental results are found to correlate well with the calculation if it is assumed that the clusters have a face-centered cubic structure with a constant number of intersecting stacking faults. Additional confirmation is obtained for the decisive role of the kinetic factor in the formation of the crystalline phase of the clusters. Conjectures are offered concerning the possible reasons why the densitometer traces presented by M-F. de Feraudy, G. Torchet, and B. W. van de Wall at the conference ISSPIC (1998) showed no substantial changes when the cluster size was increased by more than an order of magnitude.

Influence of the size effect in the exciton energy spectrum on exciton relaxation in rare gas clusters

The influence of the size effect in the exciton energy spectrum on exciton relaxation in argon, krypton, and xenon clusters is investigated by the method of VUV cathodoluminescence spectroscopy. It is found that as the average cluster size is decreased to a certain critical value Nc∼160–190 atoms/cluster a blocking of the usual one-phonon channel of exciton energy relaxation occurs when the increasing distance between successive excitonic levels in the band reaches the phonon Debye energy. This leads to the appearance of another channel of relaxation (suppressed in the solid) that ends in the desorption of excited atoms and molecules from rare gas clusters.

Features of the x-ray bremsstrahlung in the scattering of intermediate-energy electrons on atoms of inert elements

The results of experimental and theoretical studies of the differential x-ray bremsstrahlung spectra emitted in the scattering of intermediate-energy electrons on atoms of inert elements are presented in generalized form. The experimental studies were done using the low-temperature gas-jet method of generation of electromagnetic radiation. The features of the bremsstrahlung and the relationships governing the novel form of bremsstrahlung due to polarization effects are established. It is shown that the Born approximation is inapplicable for considering the bremsstrahlung cross sections in the scattering of electrons of intermediate energies on atoms of inert elements.

Investigation of Nonequilibrium Argon Condensation In Supersonic Jet By Mass-Spectrometry, Electron Diffraction and VUV Emission Spectroscopy

Even the first studies of VUV emission spectra from an electron excited supersonic argon jet discovered an important peculiarity, namely, a strong dependence of the jet spectrum composition and intensity on the stagnation pressure Po and temperature To (Fig.1).