Role of local translation in the vibrational relaxation of matrix-isolated guest molecules

We have studied the participation of local translation in the vibrational relaxation of asymmetric diatomic molecules trapped in a rare gas matrix. For a guest molecule surrounded by six nearest rare gas atoms in a two-dimensional array, it is shown that the role of local translation is to assist vibration-to-rotation energy transfer by producing strong rotation–local-translation coupling and to accept the energy mismatch of initial and final vibration–rotation levels. In the model, low-lying rotational levels are hindered and transformed into libration, and libration-to-rotation transitions take most of the vibrational energy. Applications to hydrogen chloride molecules in Ar show that the 1→0 and 2→1 vibrational de-excitation probabilities of HCl/Ar are significantly smaller than those of DCl/Ar. These probabilities increase only slightly with temperature in the range 9–30 K.