CHANNEL SWITCHING IN ABOVE-THRESHOLD IONIZATION OF XENON

The intensity dependence of the angular and energy distributions of ph otoelectrons is used to map the effect of channel closing in short-pul se, multiphoton ionization of xenon. At 800 nm, we observe the nine-ph oton channel to close at approximate to 3 x 10(13) W/cm(2) and the dom inant ionization path to switch from (8 + 1)-photon resonant ionizatio n via f-type Rydberg states to (9 + 1)-photon resonant ionization via g-type Rydberg states. Very low-energy electrons are observed prior to reaching the channel closing intensity, indicative of a contribution of nonresonant ionization.