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).