Channel switching in above-threshold ionization of xenon

The intensity dependence of the angular and energy distributions of photoelectrons is used to map the effect of channel closing in short-pulse, multiphoton ionization of xenon. At 800 nm, we observe the nine-photon channel to close at ≈3×10¹³W/cm² and the dominant ionization path to switch from $(8+1)$-photon resonant ionization 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.