The Role of Atomic Structure in High Intensity Multiphoton Ionization

Recent measurements of the photoelectron energy distribution for high intensity, short pulse multiphoton ionization have yielded a spectrum unexpectedly rich in well-resolved peaks. This structure apparently arises from the AC Stark shift of atomic levels into resonance with harmonics of the laser. A model is presented which takes into account the amount of time a shifted state is in resonance. Included are contributions from the temporal peak of the pulse as well as ionization from the leading/trailing edges. This model predicts a surprising spatials structure in the total ionization yield that is intensity dependent. The spatials structure is extracted from the electron spectrum and shows dramatic differences from conventional multiphoton ionization.