Anomalous vibrational excitation in the small angle scattering of He⁺ from HCl at Ε<inf>lab</inf> = 20-60 eV

Abstract

Energy loss spectra of forward scattered He+ ions in collisions with HCl are reported for collision energies (Εlab) in the range 20-60 eV. The spectra extend up to total energy transfers of about 2.5 eV, where 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 Δν transitions are observed. Three-dimensional quasiclassical trajectory calculations treating HCl as a rigid rotor for the long range ion-dipole/quadrupole He+-HCl interaction and a short range repulsion corresponding to the isoelectronic He+-Ar account for much of the energy transfer at low ΔΕ. At higher ΔΕ, the rigid rotor model is clearly inadequate and a better description of the experiment requires consideration of high lying repulsive states of HCl+ which appear to be temporarily accessed by charge transfer. © 1995 American Chemical Society.