COLLISIONAL EXCITATION OF VERY HIGH ROTATIONAL LEVELS OF HF IN SMALL-ANGLE SCATTERING WITH HE-50 EV( AT E(LAB)=25)

Energy loss distributions of He+ ions after collisions with HF molecul es in a secondary beam at laboratory collision energies in the range 2 5-50 eV show a regular structure indicating rotational excitation up t o Delta J=35 with no clear evidence for vibrational excitation. The ob served propensity for large Delta J transitions decreases with increas ing collision energy but increases with increase in scattering angle. Three dimensional quasiclassical trajectory calculations using a poten tial energy surface based on the He+-Ne interaction at short range and a damped long range ion-dipole (quadrupole) interaction reproduce all the essential features of the experimental results.