Theory of continuum mediated two-photon ionization, with applications to CH3I

Theory of two-photon ionization via intermediate dissociative states is developed. The theory is applied to the one-color two-photon ionization of CH3I for which the (3)Q(0) and (1)Q(1) dissociative states serve as intermediate resonances. Both CW and transient ionization spectra are calculated. The computed methyl iodide CW ionization line intensities are in profound disagreement with line intensities derived from two-photon zero kinetic energy electrons (ZEKE) experiments. The discrepancy may be viewed as evidence that ZEKE line-intensities are not simply proportional to the population of the ionic-core states. This extra dependence on the ionic-core states, most likely due to the existence of external ions, can be accounted for by comparing our calculations to the observed ZEKE Line intensities. The dynamics revealed by ultrashort pulsed two-photon ionization is also studied. In methyl iodide, we find that pulses capable of revealing ''real-time'' dissociative dynamics must be considerably shorter than 50 fs. (C) 1996 American Institute of Physics.