PENNING IONIZATION AND PHOTOIONIZATION ELECTRON SPECTROMETRY OF HYDROGEN BROMIDE

An electron spectrometric study has been performed on HBr using metastable helium and neon atoms as well as helium resonance photons. High resolution electron spectra were obtained for a mixed He(2(1)S, 2(3)S) beam, a pure He(2(3)S) beam, a mixed Ne(3s3P2, 3P0) beam, and for HeI VUV light. From the comparison of vibrational populations of HBr+(X, upsilon') and HBr+(A, upsilon'), formed by either He* and Ne* Penning ionization (PI) or HeI photoionization, we conclude that HBr+(X) formation by PI exhibits only little perturbation of HBr potentials, whereas HBr+ (A) formation by PI exhibits substantial bond stretching of HBr due to metastable atom attack preferably from the H end. For He(2(1)S) + HBr the X- and A-state vibrational peak shapes are substantially broader than for the He(2(3)S) + HBr case pointing to an additional, charge exchanged interaction (He+ + HBr-) in the entrance channel of the former system which is also responsible for a broad feature found at lower electron energies in the He(2(1)S, 2(3)S) induced PI electron spectra. For the first time, we have detected the low energy electrons in both the He(2(1)S) + HBr and He(2(3)S) + HBr spectra, associated with the major mechanism for the formation of Br+ ions: energy transfer to repulsive HBr** Rydberg states, dissociating to H(1s) and autoionizing Br** atoms. The HBr+(X) 2-PI-3/2:2-PI-1/2 fine structure branching ratios vary significantly with the ionizing agent in a similar way as for the isoelectronic, atomic target case krypton.