Multiple ionization of argon atoms by intense laser pulses

We have analyzed the recently measured single, double, and triple ionization of Ar by strong laser pulses in terms of an independent-electron model. An excellent fit to the highest intensity data is obtained in terms of two single-electron ionization probabilities that correspond to the initial 3p bound orbitals being in the m=0 or +/-1 substates. Their relative magnitudes are understood in terms of the tunneling of electrons that are in differently oriented bound orbits, and they both exhibit the expected approach to unity as the limiting over-the-barrier electron ejection is realized. The n-electron ionization probabilities go through maxima and crossings at the highest intensities that are well described by the independent-electron model.