Threshold photoelectron spectroscopy of HF in the inner valence ionization region

The threshold photoelectron (TPE) spectrum of hydrogen fluoride has been recorded using synchrotron radiation and a penetrating-field electron spectrometer over the photon energy range 30–52 eV encompassing the inner valence ionization region. The TPE spectrum is characterized by a dominant, broad band system and a weaker broad band system both of which are assigned to the `main lineʹ for (2sσ)−1 ionization, with some vibrational structure evident on the more intense one of these. The results of this study contrast with the findings for the TPE spectra of the inner valence ionization region of the heavier halogen acids and this is explained on the basis of differences between potential energy curves for the dicationic states. Observation of a broad, structureless band system on the low energy side of the `main-lineʹ band systems is attributed to indirect autoionization of neutral Rydberg states.