Internal ionization of Ar and Kr noble-gas cryocrystals by the excitation energy of trapped metastable excited atoms

Abstract

Local metastable excited states are found in Ar and Kr cryocrystals when He gas-discharge products are trapped in the growing cryocrystals. These states are detected by ESR and are interpreted as being local metastable excited np5(n+1)s3P2 atomic-type states in Ar and Kr cryocrystals. The study showed that the yield of the 3P2 excitations in the above process decreases with increasing temperature. Analysis of the results allows the following explanation of the observed effect to be given. Metastable excited He atoms from the He gas discharge are trapped in the growing Ar or Kr cryocrystals and transfer their excitation energy to the cryocrystal to form, in the process of internal ionization, a RG+ ion and a free electron in the conduction band, whereupon the fast (10-12 s) self-trapping reaction of a hole follows: RG+RG to RG2+. Thereafter either the dissociative recombination reaction RG2++e to RG2** to RG+RG*(3P2) or recombination RG2++e to RG+RG to produce ground-state atoms could take place. The former is likely at lower temperatures, and the latter at higher temperatures when the vibrational relaxation rate of the RG+ molecular ion increases and the mobility of free electrons in the conduction band decreases. This is the reason for the observed temperature dependence.

Electrons in rare-gas solids probed by EPR technique

“…Newly obtained results suggest a new interpretation of the earlier findings [1][2][3]. The interpretation gives insight not only into the emergence of the unstable signals but also into the absence of the g-factor matrix shift.…”

“…The measurements showed that g-factors of signals from different rare gases were equal within 0.0001. A suggestion put forward [1][2][3] about the splitting of the 3 P 2 atom levels by a strong anisotropic crystal field may, in principle, explain the absence of the matrix shift, but the g-factor in that case should be very close to the free electron g-factor: g e ¼ 2.0023 which differs from the measured quantity.…”

“…In earlier papers [1][2][3], an observation was reported of unstable EPR centers formed during a condensation of products of high-frequency discharge running in rare gases. A substrate was located at the center of an EPR microwave cavity.…”

“…No such spectra were observed with molecular gases. A set of experiments was undertaken [2,3] when the gaseous He was passed through the discharge channel while the matrix gas was supplied through a separate inlet tube bypassing the gas discharge (matrix channel). When no matrix gas was supplied through the inlet tube, no unstable centers were recorded.…”

“…This assumption is also supported by the fact that only rare-gas discharges provided the unstable spectra. A strong dependence was found and studied [3] of the central line intensity on the substrate temperature with He and other rare gases passed through the discharge and matrix tubes, respectively. The dependencies were different for different gases.…”