Spectroscopic properties and relaxation processes of impurity molecules in solids. II. Vibrational relaxations

Abstract

We have calculated the vibrational relaxation rates for HCl (DCl)/Ar matrix systems. The quantitative agreement between our calculations and experimental data is fairly good. Our approach based on the adiabatic approximation can consistently treat the eigenvalue problems and vibrational relaxation processes for diatomic molecules embedded in monatomic crystals. The adiabatic approximation is used to separate high (intramolecular vibration) and low frequency modes (molecular rotation and lattice vibration). The nonadiabatic couplings (the kinetic energy operators for the low frequency modes) induce the vibrational relaxation processes. Our numerical calculations support the mechanism proposed by Bondybey and Brus that for small hydrides molecular rotation is the dominant accepting mode. The A1g totally symmetric lattice modes in the classification by the irreducible representations of the substitutional site symmetry Oh are shown to be dominant to accept the energy mismatch between initial and final rotational levels. For the DCl/Ar system, we have found that the mixing of initial rotational levels due to the rotation-lattice vibration coupling is responsible for the vibrational relaxation. The calculated relaxation rate for DC1 is smaller than that of HCl by one order of magnitude.