HIGH-RESOLUTION THRESHOLD PHOTOELECTRON-SPECTROSCOPY OF HYDROGEN IODIDE

The threshold photoelectron spectrum of hydrogen iodide has been recor ded at high resolution (4-30 meV) using synchrotron radiation and a pe netrating-field electron spectrometer over the photon energy range 10. 2-31.0 eV. Extended vibrational structure in the first Franck-Condon g ap region between the X ((2) Pi(i)) and A ((2) Sigma(+)) states of HI has been observed and analyzed to yield improved spectroscopic consta nts for the X ((2) Pi(i)) state. The Rydberg states responsible for th e production of this structure, through resonance autoionization, are identified as [A ((2) Sigma(+))]ns sigma (1) Sigma(+) for n = 6-8. A v ibrational progression in the A ((2) Sigma(+)) band system in HI+ lead ing up to the dissociation continuum of this state has been identified for the first time, yielding spectroscopic constants for the state. T he A ((2) Sigma(+)) state of HI+ appears to be formed by an avoided cr ossing between the diabatic A ((2) Sigma(1/2)(+)) and (4) Pi(1/2) pote ntials. Spectroscopic constants are derived for the 'main' band system for 5s sigma(-1) inner-valence ionization of HI. (C) 1997 Elsevier Sc ience B.V.