Non-adiabatic transitions observed in H^-formation by dissociative electron attachment on HBr

Abstract

Differential cross sections and angular distributions of ions formed by dissociative electron attachment in HBr have been obtained, by means of a modified crossed-beam electron impact spectrometer, in the 5-10 eV energy range. The yields of H- ions associated with both Br(2P32/) and Br(2P12/) atoms exhibit two peaks in the energy range 5-7 eV and 7-10 eV, respectively. The first peaks in the 5-7 eV energy range have an angular distribution characteristic of a d sigma wave indicating that H- ions in this energy range are formed via a 2 Sigma HBr- resonant state. For the two higher peaks the angular distributions of H- ions are characteristic of d pi waves in agreement with 2 Pi HBr- resonant states. The first peak of the H- ions associated with Br(2P12/) atoms shows a non-diabatic transition from the 2 Sigma 1/2HBr- resonant state to the H-+Br(2P12/) dissociation limit which is due to a strong dynamical (d/dR) coupling between the two Omega =1/2 HBr-1 resonant states. This coupling occurs at a large internuclear separation of the H- and Br fragments as shown by the angular distribution of H- ions. A d/dR coupling calculation does not account for the experimentally measured branching ratios.