Saturation Effects in the Spatial and Energy Distributions in Short-pulse High-intensity Multiphoton Ionization

We have calculated the spatial and electron energy structure for short-pulse high-intensity multiphoton ionization taking into account saturation. Using an avoided-crossing picture, we have modelled multiphoton resonances induced by the high-intensity a.c. Stark effect. We have explicitly considered ionization due to the rising and falling edges of the laser pulse including saturation and ground-state depletion. Using only the laser-pulse duration as the adjustable parameter, our results show the dramatic modifications both to the spatial structure and to the photoelectron energy spectrum as saturation becomes important.