ANOMALOUS VIBRATIONAL-EXCITATION IN THE SMALL-ANGLE SCATTERING OF HE-60 EV( FROM HCL AT E(LAB)=20)

Energy loss spectra of forward scattered He+ ions in collisions with H Cl are reported for collision energies (E(lab)) in the range 20-60 eV. The spectra extend up to total energy transfers of about 2.5 eV, wher e the intensity is 6 orders of magnitude less than the elastic forward peak. At large energy transfers a number of broad maxima indicating a propensity for large Delta v transitions are observed. Three-dimensio nal quasiclassical trajectory calculations treating HCl as a rigid rot or for the long range ion-dipole/quadrupole He+-HCl interaction and a short range repulsion corresponding to the isoelectronic He+-Ar accoun t for much of the energy transfer at low Delta E. At higher Delta E, t he rigid rotor model is dearly inadequate and a better description of the experiment requires consideration of high lying repulsive states o f HCl+ which appear to be temporarily accessed by charge transfer.