Rotational energy transfer in hydrogen halide molecules at supersonic beam velocities

Abstract

Rotational energy transfer cross sections are calculated for HCl–HCl and HCl–HF collisions using classical trajectories. The exponential model for the rotational results is compared with transitions j→j′ in collisions of a supersonic HCl molecular beam with a thermal beam. Reasonable agreement with the functional form of that model is obtained. The trajectory results indicate that some modification is needed in the exponential formula, to account for microscopic reversibility in rotor–rotor collisions. Using a simple intermolecular potential without adjustable parameters, in which the only anisotropic terms are of the dipolar and dispersion type (i.e., long range), reasonable agreement is obtained with the experimental constant for HCl–HCl in the exponential. Calculations are also described for HCl–HI and HCl–HBr collisions. Microscopic reversibility for the averaged cross sections is discussed in connection with the exponential model.