Negative ion formation, vibrational excitation, and transmission spectroscopy in hydrogen halides

Abstract

Three independent experiments are performed in order to elucidate low-energy electron collisions on hydrogen halides. The formation of Cl− and Br− shows a steep onset near threshold and structure at higher energies. This structure occurs at the energies of vibrational levels of the X 1Σ+ ground state of the hydrogen halide. The transmission spectrum of HBr shows structure at the position of the v=1 level of the X 1Σ+ state. In addition, transmission spectra show that the vibrational cross section to v=1 in all the hydrogen halides rises steeply near threshold and levels off about 60 meV above threshold. A trapped-electron experiment in HCl substantiates this observation and in addition, gives the order of magnitude of the vibrational cross section near threshold (2×10−16–2×10−15 cm2).