Classical treatment of rotational relaxation in He−H 2 at low temperatures

Abstract

Rotational relaxation times and collision numbers of hydrogen in a bath of He atoms are calculated by means of a classical trajectory method over a range of temperatures from 50 to 300 K using two different choices for the initial rotational energy distribution of the molecules. In both cases the rigid rotator approximation is made, but in the first choice the initial distribution of rotational energies for each temperature is given by the classical Boltzmann distribution while for the second choice the initial distribution is chosen to be the quantized distribution of rotational energy levels. It is found that for both choices of the initial distribution of rotational energies the rotational relaxation times and rotational collision numbers are increasing functions of temperature. This temperature dependence is the opposite of that found in quantum mechanical calculations and in experiment. We conclude that it is invalid to use classical mechanics to discuss rotational relaxation in hydrogen for temperatures below 300 K.