E. V. Savchenko

Infrared absorption spectrum of matrix-isolated noble-gas hydride molecules: Fingerprints of specific interactions and hindered rotation

Noble-gas hydride molecules with the general formula HNgY (Ng denotes noble-gas atom and Y denotes electronegative fragment) are usually prepared in solid noble gases. In many cases, the matrix-isolated HNgY molecules show a characteristic structure of the H-Ng stretching absorption: A close doublet as the main spectral feature and a weaker satellite at higher energy. This characteristic band structure is studied here for matrix-isolated HXeBr and HKrCl molecules. Based on the experimental and theoretical results, we suggest a model explaining the common features of the band structure of the HNgY molecules in noble-gas matrices. In this model, the main doublet bands are attributed to matrix sites where the splitting is caused by specific interactions of the embedded molecule with noble-gas matrix atoms in certain local morphology. The weaker blueshifted band is probably a fingerprint of hindered rotation (libration) of the embedded molecule in the lattice. This librational band has a mirror counterpart at lower energies appearing at higher matrix temperatures. Our present ab initio calculations for the one-to-one Xe...HXeBr complexes and the simulation of hindered rotation in a matrix support this image.

Electronically induced defect formation in Xe-doped solid Kr

Abstract Lattice defect formation induced by excitation of excimer-type molecular centers in the Kr matrix is studied by the luminescence VUV spectroscopy method. The samples are excited by slow electrons. It is established that the trapping of electronic excitations in the regular lattice regions produces the formation of permanent point defects. The experimental data on the “ground state” mechanism of defect formation associated with radiative decay of electronic excitations and some supporting evidence for the existence of an “excited state” mechanism are obtained. The efficiency of the latter mechanism is found to correlate with the lifetime of electronic excitation of the molecular centers. This mechanism is shown to be electronically thermal. A model of defect creation and stabilization in the excited state is proposed which consists in that the molecular center is displaced to a noncentrosymmetric position followed by its reorientation. The model parameters for the center Kr 2 * are estimated.

Stability of Charge Centers in Solid Ar

The technique of thermally stimulated exoelectron emission (TSEE) was used for the first time for a study of thermal stability of charged centers in solid Ar. The data obtained demonstrate the efficiency of combining the TSEE study with thermally stimulated luminescence (TSL) for trap-level analysis. High thermal stability of charge centers at low temperatures was found.

Relaxation processes induced by radiative electronic transitions in preirradiated rare gas solids

Abstract Energy relaxation processes in films grown from pure Ar gas and O2–Ar mixtures preliminary irradiated by an electron beam were studied with a focus on the role of radiative electronic transitions in the relaxation cascades. A combination of luminescent spectroscopy methods (thermally stimulated luminescence (TSL) including spectrally resolved TSL and cathodoluminescence) with thermally stimulated exoelectron emission technique was used. New additional data were obtained in a favor of the radiative mechanism of electronic relaxation processes––electron detrapping and transport followed by exoelectron emission and charge recombination. Correlation in the electron and in the low temperature desorption yields is clearly demonstrated. Suppression of the desorption from oxygen-containing films was found in contrast to the effect observed under excitation.

Exciton-induced lattice defect formation

The lattice defect formation induced by electronic excitation in solid Ne is studied using the selective vacuum ultraviolet spectroscopy method. The samples are excited with synchrotron radiation in the range of excitonic absorption n=2Γ(3/2). The dose dependence of the intensity distribution in the band of atomic-type self-trapped exciton luminescence is analyzed. Direct evidence of the formation and accumulation of point lattice defects in solid Ne via the excitonic mechanism is obtained for the first time. A model of permanent lattice defect formation is discussed.

Proton solvated by noble-gas atoms: simplest case of a solvated ion

The solvation of a proton by up to six rare-gas atoms He, Ne, Ar, Kr, and Xe is investigated by B3LYP density functional theory with large basis sets, forming the first systematic study of all rare gases, He through Xe, on the same high level of theory. The solvation energy for regular two-fold, trigonal planar, tetrahedral and octahedral coordination shows, as known previously, that the protonated rare gas dimer is the most stable configuration in every case. Solvation of a point charge by hard polarizable spheres yields the same preference for two-fold coordination. Two rare gas atoms shield the proton efficiently, and additional rare gas atoms may be coordinated in an equatorial plane or along the axis of the central protonated rare gas dimer, with binding energies and bond lengths comparable to those of the corresponding rare gas solids. The influence of additional solvent atoms on the harmonic stretching frequencies is minor and cannot explain the large shift observed in low temperature matrices. Proton diffusion is examined by calculating the transition state for isomerization of Rg3H+ species, which yields barrier heights of 8.8, 11.5, 29.7, 32.3, and 35.5 kJ mol-1 for He, Ne, Ar, Kr, and Xe, respectively. Geometries, harmonic frequencies, bond dissociation energies and partial charges of mixed protonated rare gas dimers reveal a consistently smooth trend of these properties with size and polarizability of the rare gas atoms. Based on these findings, the assignment of spectral lines attributed to the mixed ArH+Kr, ArH+Xe, and KrH+Xe species is questioned. The stabilization of positive charge centers in solid Xe in the presence of hydrogen atoms is also discussed.

Lattice rearrangement induced by excitons in cryocrystals

Experiments on permanent lattice-defect formation induced by electronic excitation of rare-gas solids are presented. The creation of triplet excitons Γ(3/2) is found to be the primary process, followed by the exciton self-trapping. Probable models of defect formation are discussed.

Spectroscopy of Ionic Centers in Solid Xe

AbstractIntrinsic ionic centers in solid Xe were studied by absorption and laser-induced fluorescence spectroscopy. The influence of the temperature, of lattice defects and of impurities were examined. Based on the analysis of the spectra in comparison with theoretical data the revealed bands were assigned to the transition between the ground state and the lower excited states of the

Electron-hole recombination induced desorption of excimers from solid Ar

Xe_2^ +