Threshold photoelectron spectroscopy of HCl up to 40 eV

Abstract

The threshold photoelectron spectrum of HCl has been recorded using synchrotron radiation over the energy range encompassing both outer and inner valence ionization. A large enhancement in the intensity of the v+ = 0–1 bands of the HCl+ 2∏32 subsystem was observed in comparison with the same vibrational bands of the 2∏12 subsystem which are attributed to autoionization processes. Resonance autoionization was also found to be responsible for the populating of upper vibrational levels (v+ = 2–14) of the X 2∏i system in the Franck-Condon gap region preceding the onset of the A 2∑+ band system. The observation of vibrational-band broadening for v+ = 7–10 of the A 2∑+ system is attributed to combined autoionization processes between repulsive neutral Rydberg states and bound vibrational levels of the A 2∑+ state and the continuum of repulsive ion states, the latter of which are responsible for predissociation in the A 2∑+ state of HCl+. In the inner valence ionization region (20.0–40.0 eV), pronounced resonance structure was observed which correlates well with the calculated energy positions of 4σ−1 satellite ions states of HCl. In addition, the direct populating of repulsive ion states in the 20.0–25.2 eV range is suggested by the observation of several broad, structureless band features, and the detection of three vibrational bands in this same energy range suggests the presence of a bound neutral Rydberg state that undergoes autoionization into the continuum of a repulsive ion state.