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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 6×1013 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 Schrödinger 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.