Channel-closing-induced resonances in the above-threshold ionization plateau

Multiphoton resonances with ponderomotively upshifted Rydberg states are believed to have dramatic effects on the plateau of high-order above-threshold ionization. For short pulses and intense laser fields, individual Rydberg states lose their physical significance. Under these conditions, on the basis of experimental and theoretical investigation of the dependence of the photoelectron spectra on the laser intensity, we propose that the observed effects can be largely understood in terms of the channel closings that are characteristic of a short-range model potential.