Photoelectron spectroscopy of the multiphoton ionization and above threshold ionization of HI at 532 and 355 nm

Multiphoton ionization and fragmentation of the hydrogen iodide molecule have been examined at 355 and 532 nm and optical field intensities of 10 ¹¹-5×10¹² W cm^-2 by photoelectron spectroscopy and time-of-flight mass spectrometty. At 532 nm, scission of the H-I bond is favored over direct photoionization of the molecule. Three and four photon ionization of HI at 355 nm (ℏω≈3.5 eV), however, reveals clear vibrational progressions associated with the HI⁺ 2Π_3/2 and 2Π_1/2 ion product states. Analysis of these, spectra yields vibrational frequencies for the ²Π _3/2 and ²Π_1/2 states that agree to better than 8% with those determined by Böwering et al. [Chem. Phys. Lett. 189, 467 (1992)] from single photon ionization experiments. Above threshold ionization (ATI) of both HI and atomic iodine are also observed.