Two- and three-photon ionization of rare gases using femtosecond harmonic pulses generated in a gas medium

The fifth harmonic of a 60-fs 10-Hz terawatt titanium:sapphire laser generated in Xe gas is used to ionize Xe and Kr atoms by nonresonant two-photon absorption and At atoms through a quasiresonant three-photon process. At relatively low harmonic yields, the variation of the number of Kr and Xe ions with the number of harmonic photons is quadratic. At high harmonic yields, it deviates from a quadratic behavior, which we attribute to the influence of ionization in the harmonic process distorting the harmonic wave front. The two-photon ionization cross sections in Xe and Kr at 160 nm are found to be comparable