Above-threshold ionization contrast and channel closure in argon

Above-threshold ionization (ATI) electron energy spectra of argon with 800 nm, 40 fs pulses display a contrast which goes through a sharp minimum around 6 x 10(13) W cm(-2). The contrast change encompasses the whole ATI spectrum including the two plateaux. The results of a full numerical solution of the time-dependent Schrodinger equation and the integration over the interaction volume show that in the low-energy part of the spectrum this effect is due to the closing of the 12-photon ionization channel and the correlated opening of the possibility for 12-photon resonances with a different parity. However, the calculation suggests a different origin for the substructure observed in the plateau.