PHOTOELECTRON-SPECTROSCOPY OF THE MULTIPHOTON IONIZATION AND ABOVE-THRESHOLD IONIZATION OF HI AT 532-NM AND 355-NM

Multiphoton ionization and fragmentation of the hydrogen iodide molecu le have been examined at 355 and 532 nm and optical field intensities of 10(11)-5x10(12) W cm(-2) by photoelectron spectroscopy and time-of- flight mass spectrometry. At 532 nm, scission of the H-I bond is favor ed over direct photoionization of the molecule. Three and four photon ionization of HI at 355 nm (h omega approximate to 3.5 eV), however, r eveals clear vibrational progressions associated with the HI+ (2) Pi(3 /2) and (2) Pi(1/2) ion product states. Analysis of these spectra yiel ds vibrational frequencies for the (2) Pi(3/2) and (2) Pi(1/2) states that agree to better than 8% with those determined by Bowering et at. [Chem. Phys. Lett. 189, 467 (1992)] from single photon ionization expe riments. Above threshold ionization (ATI) of both HI and atomic iodine are also observed.