On the determination of HCl+ (A,v?) vibrational populations from HCl+ (A?X) fluorescence spectra: direct comparison between photoionization and penning ionization

Abstract

Using a beam apparatus, we have measured the HCl+ (A,v′→X,v″) fluorescence spectra of HCl+ (A,v′) ions formed in HeI (58.4 nm), and NeI (73.6 nm) photoionization and, for the first time, in He (23S) Penning ionization under single collision conditions with a wavelength bandwidth around 1 nm. In addition, we have studied Ne (3s3P2, 0) Penning ionization of HCl at three different collision energies. The procedure and the problems in extracting HCl+ (A,v′) vibrational populations from the data are discussed in some detail. Thedirect comparison of photoionization and Penning ionization data allows definitive conclusions to be drawn on the question whether final state interactions in the Penning reaction change the “nascent” vibrational population (determined by electron spectrometry); for He (23S)+HCl, such changes are shown to be absent within the experimental uncertainty (<±10%). For Ne (3s3P2, 0)+HCl, the HCl+ (A,v′=0, 1) populations are also found to be close to those measured by electron spectrometry and essentially independent of collision energy in the range 34–96 meV. From measurements of the fluorescence intensity as a function of HCl density, we have evidence for a fast loss of HCl+ (A,v′) ions in collisions with HCl (rate constant around 5·10−9 cm3s−1).