Threshold photoelectron spectroscopy of HCl and DCl Original Research Article

We have measured the threshold photoelectron spectra of HCl and DCl with varying resolution (3–40 meV) using tunable synchrotron radiation and employing the penetrating-field electron-spectroscopic technique over a wide photon energy range (up to 34 eV) that includes the inner-valence ionization region in the molecules. The observed spectra display features produced by both direct ionization and autoionization processes. Autoionization is found to play a particularly important role in the outer-valence ionization region of these molecules. For example, in the formation of the v+=0 band of the X (2Π3/2) subsystem, in the populating of high vibrational levels of the X (2Πi) system in the Franck–Condon gap region between the X (2Πi) and A (2Σ+) states, and in the predissociated levels of the A (2Σ+) state. The production of extended vibrational structure in the X (2Πi) system is attributed to resonance autoionization of 4sσ, 5sσ and 6sσ 1Σ+ Rydberg states. These and other threshold photoelectron results suggest a propensity for threshold electron production from sσ Rydberg states. Photoionization of HCl and DCl in the inner-valence ionization region between 25 and 30 eV yields nine vibrationally resolved band systems that include the `main lineʹ 3sσ−1 band system and four sets of pairs of vibrationally very similar satellite ion systems. Absolute vibrational numbering in these systems was greatly facilitated by having the isotopomer threshold photoelectron spectra.