Luminescence evidence for bulk and surface excitons in free xenon clusters

Cathodoluminescence spectra of free xenon clusters produced by condensation of xenon-argon gas mixtures in supersonic jets expanding into vacuum were studied. By varying initial experimental parameters, including xenon concentration, we could obtain clusters with a xenon core (300-3500 atoms) covered by an argon outer shell as well as shell-free xenon clusters (about 1500 atoms). The cluster size and temperature (about 40 K for both cases) were measured electronographically. Luminescence bands evidencing the existence of bulk and surface excitons were detected for shell-free xenon clusters. The emission from bulk excitons in small clusters is supposed to be due to processes of their multiple elastic reflections from the xenon-vacuum interface. A presence of an argon shell causes extinction of the excitonic bands. In addition, some new bands were found which have no analogs for bulk xenon cryosamples.Comment: The final modified version will be published in Phys. Rev. A 76 (2007

Photoproduction of Long-Lived Holes and Electronic Processes in Intrinsic Electric Fields Seen through Photoinduced Absorption and Dichroism in Ca_3Ga_{2-x}Mn_xGe_3O_{12} Garnets

Long-lived photoinduced absorption and dichroism in the Ca_3Ga_{2-x}Mn_xGe_3O_{12} garnets with x < 0.06 were examined versus temperature and pumping intensity. Unusual features of the kinetics of photoinduced phenomena are indicative of the underlying electronic processes. The comparison with the case of Ca_3Mn_2Ge_3O_{12}, explored earlier by the authors, permits one to finally establish the main common mechanisms of photoinduced absorption and dichroism caused by random electric fields of photoproduced charges (hole polarons). The rate of their diffusion and relaxation through recombination is strongly influenced by the same fields, whose large statistical straggling is responsible for a broad continuous set of relaxation components (observed in the relaxation time range from 1 to about 1000 min). For Ca_3Ga_{2-x}Mn_xGe_3O_{12}, the time and temperature dependences of photoinduced absorption and dichroism bear a strong imprint of structure imperfection increasing with x.Comment: 20 pages, 10 figure

EXCITON - INDUCED PROCESSES IN SOLID MOLECULAR OXYGEN - NEON SOLUTIONS

Author Institution: B. Verkin Institute for Low Temperature Physics and Engineering, Ukrainian Academy of SciencesThe paper is concerned with the processes induced by the trapping of excitons of the neon matrix by impurity centres O and Xe. Binary and triple solutions are studied by a cathodoluminescence spectroscopy method. It is found that the exciton trapping by impurity particles results in an effective dissociation of O molecules, causes an accelerated migration of atoms through the the crystal and a desorption of O, initiates a defect formation in the vicinity of impurity centers and produces a formation of excimer compounds XeO in triple solutions Ne-O -Xe. The mechanisms of these processes associated with the transport of energy of the matrix electronic excitations to an impurity subsystem and its further relaxation are considered. It is shown that all the effects are based on the general principles conditioned by the energy structure and thermophysical characteristics of the Ne matrix- (i) the fast dissipation of exciton excess energy followed by lattice local heating and deformation; (ii) the repulsive behavior of the interaction between Rydberg’s excited impurity center and surrounding atoms of the matrix

SPECTROSCOPY STUDY OF DEFECT PRODUCTION IN THE EXCITED MOLECULAR STATE IN RARE GAS SOLIDS.

$^{1.}$ I. Ya. Fugol, A. N. Ogurtsov, O. N. Grigorashchenko, and E. V. Savchenko Sov. J. Low Temp. Phys. 18(1), 27, (1992).Author Institution: Derartment of Spectroscopy, B. Verkin Institute for Low Temperature Physics and Engineering, Ukrainian Academy of SciencesElectronic excitation of Solid Rare Gases induces substantial lattice distortions. One of these is the formation of an excited state dimer $R^{\ast}_{2}$ in a regular lattice. The data have been obtained which suggest that the self-trapping into molecular states results in a production of $defects^{1}$. In this paper we present recent results on creation and modification of structural defects during formation of molecular centers $R^{\ast}_{2}$ and after their dissociation. The experiments were performed under low temperature irradiation by an electron beam of subthreshold energy. The defects were detected by means of vacuum ultraviolet luminescence spectroscopy. Dose dependencies of emission spectra of $R^{\ast}_{2}$ were studied in a temperature range where the thermal fluctuation processes of defect formation and diffusion were ``frozen out''. A build-up of stable point defects in samples was found. Based upon the analysis of the temperature and dose dependencies we have proposed two mechanisms of defect formation induced by trapping into dimer states: excited state molecule mechanism and ground-state molecular one. It is assumed that the off-center displacement of excted state dimers gives rise to an irreversible lattice distortion prior to radiative decay. The local distortions persist in the form of structural defects after annihilation of electronic excitations. It is shown that the defect formation processes occure in long-lived electronic state $^{3}\Sigma^{+}_{u}$

Kinetic study of inelastic radiation-induced processes in rare-gas cryocrystals

The processes of large-scale atomic displacements induced by exciton self-trapping were studied using the selective vacuum ultraviolet luminescence method. Models of Frenkel pairs creation in rare-gas crystals are discussed with a focus on excited-state mechanisms of defect formation. A simple kinetic model of defect accumulation in rare-gas samples is proposed

Photodissociation of water in rare gas matrixes: cage effect and local heating of the lattice

The H fragment produced in the photodissocn. of water (H2O -> H + OH) is lighter than the lattice atoms and a local heating occurs over long time scales and large spatial regions. Therefore, use of a continuum model for the lattice is justified. The local heating is promoted by a decreasing heat cond. with increasing temp. Solns. of a nonlinear heat cond. equation show that the heat release inside the cage leads to a temp. increase in its nearest surroundings up to the m.p. in argon. Melting enables the light fragment to overcome the cage barrier. In the case of krypton, the thermal effect is less pronounced, while in Xe, there is none. The above results are in qual. agreement with exptl. data. [on SciFinder (R)