Threshold photoelectron spectroscopy of HBr and DBr Original Research Article

The threshold photoelectron spectra of HBr and DBr have been recorded with varying resolution (3–20 meV) over a wide photon energy range (11.5–31.5 eV) using synchrotron radiation and a penetrating-field electron spectrometer, encompassing both the outer- and inner-valence ionization regions. Extensive vibrational structure has been observed in the X(2Πi) band systems for both isotopomers in the Franck–Condon gap region, between the X(2Πi) and A(2Σ+) states, that is attributed to the resonance autoionization of 5sσ, 6sσ and 7sσ 1Σ+ Rydberg states. Resonance autoionization is also found to play a role in the vibrational bands within the A(2Σ+) system that is associated with both non-predissociated and predissociated vibrational levels of the A(2Σ+) state. This is evident by the observed increased broadening of these vibrational bands in comparison to those formed in conventional photoelectron spectroscopy obtained at a similar resolution. Photoionization of HBr and DBr in the inner-valence ionization region between 23 and 29 eV shows four sets of pairs of very similar, vibrationally resolved band systems that are identified as being due to satellite ion states and a single, vibrationally resolved, band system assigned to the `main-lineʹ 4sσ−1 ionization. Absolute vibrational numbering in these systems was greatly facilitated by having the isotopomer threshold photoelectron spectra. Evidence for the direct excitation of repulsive ion states in the 17.5–23.0 eV range has been found.