Multiphoton ionization and photoelectron spectroscopy of hydrogen iodide with fixed wavelength and tunable laser sources

Abstract

Multiphoton ionization of the hydrogen iodide molecule has been examined by photoelectron spectroscopy. The fixed wavelength experiments, at 355 nm and optical intensities of $\sim$10$\sp{12}$ W/cm$\sp2,$ reveal clear vibrational progressions associated with the $X\sp2\Pi$ spin-orbit split ion states following three- and four-photon ionization. Analysis of these spectra yields vibrational frequencies that differ from those from single photon experiments of Bowering et al. in ref. 27. These differences are explained by a differing set of photoelectron peak assignments for the high $\nu\sp+$ members of both series. Tunable laser experiments, at optical intensities of $\sim$10$\sp{11}$ W/cm$\sp2,$ have demonstrated (2 + 1)-REMPI ionization of the HI molecule through resonances in the intermediate $b\sp3\Pi\sb2$(v = 0,1;J = 0-12) state. However, resonances with Rydberg series in the ionization continua between the $X\sp2\Pi$ manifolds seen in single photon ionization experiments by Carlson et al. in ref. 20 were not seen here. The lack of any structure in this region in the present experiments is therefore attributed to resonances with predissociated states in the continua, which are favored over those states accessed in the single photon experiments.