Incorporation of a wave-packet propagation method into the S-matrix framework: Investigation of the effects of excited state dynamics on intense-field ionization

We propose a theoretical method for investigation of ionization of atoms and molecules in intense laser fields that copes with the effects of excited state dynamics (or intramolecular electronic dynamics). The time-evolving wave packet composed of only bound electronic states, ∣Φ_i(t)⟩, is introduced into a framework of the intense-field S-matrix theory. Then, the effects of both Coulomb field and radiation field on the bound electron(s) are well described by ∣Φ_i(t)⟩, while the effects of a radiation field on a freed electron are also treated in a nonperturbative way. We have applied the theory to ionization of H and H₂⁺ in ultrashort intense laser pulses. Although only a small number of Gaussian functions are used in the expansion of ∣Φ_i(t)⟩, the present method can quantitatively reproduce the features of enhanced ionization of H₂⁺ obtained by an accurate grid propagation method. This agreement supports the view that field-induced population transfer between the lowest two electronic states triggers the enhancement of ionization at large internuclear distances. We also applied the method to calculate the photoelectron momentum distribution of H in an intense near-infrared field. A broad low intensity component due to “rescattering” appears in the distribution of the momentum perpendicular to the polarization direction of an applied laser field, as observed in the experiments of single ionization of noble gas atoms. The present method provides a practical way of properly describing the nonperturbative nature of field-induced dynamics of an electron (or electrons) in the presence of both Coulomb and radiation fields.