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

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( 2 P 3 2/) and Br( 2 P 1 2/) 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( 2 P 1 2/) atoms shows a non-diabatic transition from the 2 Sigma 1 /2HBr - resonant state to the H - +Br( 2 P 1 2/) 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.