Chattering collisions and their effects on gas phase rotational energy relaxation cross sections

Rotational energy relaxation cross sections, σR , for nitrogen in dilute atomic gases (He, Ne, Ar, and Xe) are calculated by classical trajectory simulations for a variety of simple potentials and from the Boltzmann equation for hard convex bodies in the single impulse collision approximation. The single impulse approximation, which ignores multiple impulse (chattering) collisions, is reasonably adequate to describe linear and angular momentum relaxation, but not rotational energy relaxation. For the light noble gases, the hard body derived results together with the small chattering corrections suffice to fit σR . It is also the case that chattering collisions markedly decrease σR from the single impulse approximation value for Xe--N2 in the hard-body model. However, the value of σR experimentally measured and calculated by Kistemaker and de Vries using a soft potential is considerably less than that obtained from hard-body models including chattering. Addition of a square well attractive potential of the appropriate shape to the hard core potential leads to the desired decrease in σR with well depth in accord with both the trajectory calculations and experiments of Kistemaker and de Vries. The reasons are analyzed