High-resolution threshold photoelectron spectroscopy of hydrogen iodide

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 ((2) Pi(i)) and A ((2) Sigma(+)) states of HI+ has been observed and analyzed to yield improved spectroscopic constants for the X ((2) Pi(i)) state. The Rydberg states responsible for the production of this structure, through resonance autoionization, are identified as [A ((2) Sigma(+))]ns sigma (1) Sigma(+) for n = 6-8. A vibrational progression in the A ((2) Sigma(+)) 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 ((2) Sigma(+)) state of HI+ appears to be formed by an avoided crossing between the diabatic A ((2) Sigma(1/2)(+)) and (4) Pi(1/2) potentials. Spectroscopic constants are derived for the 'main' band system for 5s sigma(-1) inner-valence ionization of HI. (C) 1997 Elsevier Science B.V.