FLUORESCENCE STUDY OF DEFECT FORMATION via EXCITON TRAPPING INTO MOLECULAR STATES 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). 2. I. Ya. Fugol', B.V. Savchenko, A. N. Ogurtsov and O.N. Grigorashchenko, Physica B. 190, 347 (1993)Author Institution: Department of Spectroscopy, B. Verkin Institute for LOW Temperature Physics and Engineering, Ukrainian Academy of SciencesInformation on defects is very important for elucidation of mass diffusion and chemical reactions in solids. Recently we reported on the pioneering observation of defect formation induced by exciton self-trapping into molecular states in Rare Gas Solids. The paper reports physics of this phenomenon is based on the trapping of electronic excitation energy by the lattice and its conversion to the kinetic energy of atomic motion. The paper reports new data on molecular hosts in Kr matrix. The electronic subsystem was excited by an electron beam of subthreshold energy at low temperatures. So, the conditions of the experiments excluded the classical knock-on and thermally-activated mechanisms of defect formation. The defects were detected by means of vacuum ultraviolet fluorescence spectroscopy. The experimental study of time evolution of fluorescence spectra revealed a build-up of stable point defects in the sampled. It is shown that the defect formulation is induced by exciton trapping into molecular states of both the matrix and the impurity centers. The efficiency of defect formation as a factor of the parameters of electronic states of molecular centers was analyzed. The excited state mechanism which involves the off-center displacement of molecular center was verified. Based on the experimental data the electronically induced mechanism of mass diffusion was suggested

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 $^{3}\Sigma^{+}_{u}$

CAMERON BANDS IN THERMOLUMINESCENCE OF CO DOPED SOLID AR.

Author Institution: B. Verkin Institute for Low Temperature Physics and Engineering, 47 Lenin Avenue, Kharkov 310164, UkraineThe emission spectra of CO doped solid Ar were obtained in the temperature range 5-30K during the thermoluminescence and cathodoluminescence measurements using the low energy electron excitation. The changes of the bands shape due to variation in the sample growth conditions were studied. Thermally activated emission of the Cameron bands was observed for the first time. Applying a deconvolution of the thermoluminescence glow curves, parameters of single glow peaks were determined. Within the Randall-Wilkins approximation, solid argon electron traps depths were obtained by the numerical fitting of the glow curves

ULTRAVIOLET EMISSION OF INTRINSIC MOLECULAR IONS IN SOLID AR.

1. H. Langhoff, Opt. Commun. 68, 31 (1988). 2. W. Krotz, A. Ulrich, B. Busch, G. Ribitzki, J. Wieser, Phys. Rev. 443, 6089 (1991). 3. E. V. Savchenko, A.N. Ogurtsov, O.N. Grigorashchenko, S.A. Gubin, Chem. Phys. 189, 415 (1994).Author Institution: B. Verkin Institute for Low Temperature Physics and Engineering, 47 Lenin Avenue, Kharkov 310164, UkraineExcitation of Rare Gas Solids (RGS) results in emission of molecular continua in vacuumultraviolet (VUV) and ultraviolet (UV) spectral regions. The well-known VUV band centered at 9.7 eV is attributed to the radiative decay of self-trapped excitons of molecular type ${Ar^{\ast}}_{2}$. Origin of the UV continum centered at 6.2 eV is still under consideration. The paper reports new data on UV emission of solid Ar and give new assignment of this spectral feature. The samples with variable content of defects were grown in a special low temperature cell. Solid Ar was excited by low-energy electrons. The influence of oxygen-containing impurities was studied. Temperature dependence of the emission spectra and dose dependences at fixed temperatures were investigated. Glow curve of solid Ar was observed for the first time. Analysis of the glow curve assuming the first order kinetics was performed. UV and VUV emissions of solid Ar were measured as a function of electron trap concentration and the recombination of intrinsic molecular ions was established. Analysis of our results and comparison of those with the gas phase $data^{1,2}$ permit us to suggest that UV emission of solid Ar is due to the decay of the molecular ions ${Ar_{2}}^{++}$. The data obtained reveal a new channel of electronically induced defect formation in RGS in addition to those founded $earlier^{3}$

Thermally stimulated exoelectron emission from solid Xe

Thermally-stimulated emission of exoelectrons and photons from solid Xe pre-irradiated by low-energy electrons were studied. A high sensitivity of thermally-stimulated luminescence (TSL) and thermally-stimulated exoelectron emission (TSEE) to sample prehistory was demonstrated. It was shown that electron traps in unannealed samples are characterized by a much broader distribution of trap levels in comparison with annealed samples and their concentration exceeds in number that in annealed samples. Both phenomena, TSL and TSEE, were found to be triggered by release of electrons from the same kind of traps. The data obtained suggest a competition between two relaxation channels: charge recombination and electron transport terminated by TSL and TSEE. It was found that TSEE predominates at low temperatures while at higher temperatures TSL prevails. An additional relaxation channel, a photon-stimulated exoelectron emission from pre-irradiated solid Xe, was revealed

Exciton - induced lattice defect formation

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