Rotational fine structure in the UV photoelectron spectra of HF and HCl

Abstract

He I and Ne I excited photoelectron spectra of the HF and HCl molecules have been recorded at a photoelectron linewidth (FWHM) of 2.5–3.5 meV. At this resolution much of the rotational structure has been resolved in the photoelectron bands associated with the X 2II state of HF+ and the A 2∑+ states of HF+ and HCl+. For the X 2II state of HCl+ a marked influence from the rotational structure has been observed on the line profile. From these spectra the following adiabatic binding energies have been obtained: 16.0460 eV (X, HF+), 19.1154 eV (A, HF+), 12.7447 eV (X, HCl+) and 16.2678 eV (A, HCl+). A detailed analysis of the rotational structure has been made for HF by comparison with the results of recent ab initio calculations. A good overall agreement is found between the experimental and theoretical results. The experimental results indicate the presence of a dissociation barrier in the potential curve of the A state that is 50 meV in HF+ and 60 meV in HCl+. For the A state of HCl+ the rotational structure disappears in the range of v = 8,9 and 10, suggesting a lifetime in this range of about 10−13 s. This lifetime limitation is associated with an interaction with repulsive states leading to predissociation at the location of the curve crossings.