Calculations of High Intensity Multiphoton Ionization and Photoemission from Atoms*

A surprising array of effects have been observed during high-intensity, short-pulse laser excitation of atoms and molecules. To model these measurements, methods have been developed to solve the time-dependent Schrödinger equation for an atom in a time varying, classical electromagnetic field. Studies on many atomic and molecular systems have been performed over a range of intensities from the regime within which perturbative techniques are valid up to field strengths well above an atomic unit (I > 3.51x1016 W/cm2). These calculations have provided predictions for ionization rates, photoelectron energy and angular distributions and photoemission rates. The effects of ac Stark shifted and intensity broadened intermediate states on the emission processes have been investigated. At high frequency and the highest intensities atoms are found to undergo a transition to a state or states which are surprisingly stable with respect to ionization. This had been predicted using a Floquet approach which did not allow for the time variation of the pulse envelope. The time-dependent calculations, however, show that an appreciable fraction of the electronic wave function can survive the rapid rise of an intense pulse and become stabilized with a greatly reduced ionization rate. Here we present a discussion of some of the most recent results.