Vibrational relaxation of HCl with inert gas collision partners

The vibrational relaxation of HCl has been measured behind incident shock waves in the presence of He, Ne, Ar, and Kr over a temperature range of 800–4100°K and a pressure range of 10–400 torr. The data show a decrease in the efficiency of energy transfer with increasing reduced mass. This tendency appears to be much stronger than predicted by present theories even when the reduced mass is assumed to be that for H atom-inert gas collisions rather than HCl-inert gas.