Failure of rigid shell models for rotationally inelastic LiH--He collisions

A simple rigid-body model for rotationally inelastic LiH--He collisions has been implemented. This treatment assumes impulsive collisions between a point particle and a smooth spherical shell whose center is displaced from the center of mass of the LiH molecule. For a given collision energy the radius and displacement of the shell are adjusted for a best fit to the equipotential contour at this energy on the ab initio surface of D. M. Silver [J. Chem. Phys. 72, 6445(1980)]. Cross sections for transitions from the rotationless j=0 state to all possible final j' states have been computed in a classical trajectory approach and have been compared to more accurate quantum coupled-states values at E/sub col/=0.3 eV [E. F. Jendrek and M. H. Alexander, J. Chem. Phys. 72, 6452(1980)]. The results from this simple model differ drastically both in magnitude and their j' dependence from those obtained in the more sophisticated treatment. While such rigid-body models have been used in the analysis of inelastic scattering experiments, the present study suggests that little physical significance can be attached to the size or shape of the rigid shell contour so obtained