Infrared absorptions of interstitial hydrogen atoms in solid argon and krypton

Infrared absorptions are reported in the spectra of inert gas-hydrogen matrix samples which were deposited after the gas mixture passed through a glow discharge. For hydrogen in argon, absorption is recorded at 90S cm^-1. This band shifts to 644 cm^-1 for deuterium in argon and no new bands are observed with hydrogen-deuterium mixtures. With krypton matrix, similar absorptions are observed at 852 cm^-1 (H₂-Kr) and at 607 cm^-1 (D₂-Kr). Prolonged infrared irradiation in the spectral range of the absorptions causes them to disappear, the hydrogen absorptions more rapidly than the deuterium features. No similar bands could be detected when xenon or neon matrixes were used. The very small ³⁶Ar-⁴⁰Ar isotopic shift (0.2±0.1 cm^-1) shows that the argon absorptions cannot be attributed to HAr, to HAr⁺ or to HAr₂⁺. The evidence is consistent with the interpretation that both in argon and in krypton, the absorptions are due to hydrogen atoms trapped in O_h interstitial sites in an undistorted inert gas lattice. This interpretation is supported by ab initio calculations for H atoms in a neon lattice. The loss on infrared irradiation is attributed to intramolecular energy transfer into local lattice vibrational motion which permits escape from the interstitial site and recombination.