A HARD SHAPE POTENTIAL MODEL FOR ATOM-SYMMETRICAL TOP INELASTIC CROSS-SECTIONS

The quantum theory of hard shape collisions has been applied to atom-s ymmetric top collisions, with particular reference to He-NH3. The hard shape radius function has been expressed by an expansion in angular f unctions and key cross section ratios have been calculated as function s of the coefficients of the leading terms. A hard shape function for He-NH3 has been derived by optimising cross sections to previous calcu lations by Billing et al.; reported to be broadly in line with experim ental measurements of relative cross sections by Schleipen et al. Ener gy and angular momentum propensity rules have been examined for the cr oss sections sigma11 --> j'k' and sigma41 --> j'k'. An exponential ene rgy gap propensity rule is satisfactory but with different parameters for different \DELTAk\. However, the cross section sigma11 --> 44 is u nexpectedly large; it appears that transitions are favored in which th e magnitude of the total angular momentum is changed but not its direc tion.