Relaxation of vibrationally excited HCl molecules in the H2O–HCl collision complex

Abstract

The temperature dependence of the relaxation of HCl(v=1) by H2O in a complex-mode collision is studied in a semiclassical approach. The de-excitation probability takes a maximum value near room temperature, and it decreases logarithmically with increasing temperature. The dependence is nearly linear. Below room temperature, the relaxation becomes less efficient. This unusual temperature dependence is a result of the vibrational relaxation occurring in complex-mode collisions, which are dominated by large impact parameter interactions. The principal pathway for the removal of vibrational energy is the H–Cl oscillatory and librational motions along the O–H–Cl configuration. When these hindered motions gain the energy, they undergo transitions to free rotational states. The decreased energy transfer efficiency at low temperature is due to the slowing of rotational motions. Energy transfer to the O–Cl large-amplitude motion is of minor importance.