High-resolution threshold photoelectron spectroscopy of hydrogen iodide Original Research Article

The threshold photoelectron spectrum of hydrogen iodide has been recorded at high resolution (4–30 meV) using synchrotron radiation and a penetrating-field electron spectrometer over the photon energy range 10.2–31.0 eV. Extended vibrational structure in the first Franck-Condon gap region between the X (2Πi) and A (2Σ+) states of HI+ has been observed and analyzed to yield improved spectroscopic constants for the X (2Πi) state. The Rydberg states responsible for the production of this structure, through resonance autoionization, are identified as [A (2Σ+]nsσ 1Σ+ for n = 6–8. A vibrational progression in the A (2Σ+) band system in HI+ leading up to the dissociation continuum of this state has been identified for the first time, yielding spectroscopic constants for the state. The A (2Σ+) state of HI+ appears to be formed by an avoided crossing between the diabatic A (2Σ12+) and 4Π12 potentials. Spectroscopic constants are derived for the ‘main’ band system for 5sσ−1 inner-valence ionization of HI.