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 $\approx 3\times {10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ and the dominant ionization path to switch from $\mathrm{}(8+1)\mathrm{}$-photon resonant ionization via $f$-type Rydberg states to $\mathrm{}(9+1)\mathrm{}$-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.