Abstract

Recently, the Above-Threshold-Ionization (ATI) phenomenon has received much attention, in particular from theoreticians bringing up models to interpret the suppression of the low-energy part of the electron energy spectrum. In this respect, the reasons why some electron peaks should be suppressed can be classified in three groups: first, intensity effects which modify, in all models, the ATI probabilities and in some of them would induce strong Stark shifts of the ionization limit. Second, polarization effects: it has been proposed that in circular polarization, the high-angular-momentum final states, which are also the high-energy part of the spectrum, should dominate over the low-angular-momentum ones. Third, space charge effects, which could prevent the low-energy electrons from escaping from the interaction region. In this paper, after a brief review of the experimental and theoretical material, we present and discuss new experimental results obtained in a regime free of space charge effects, using a magnetic collection and time-of-flight technigue. In xenon at 1064 nm, the main results are: (i) the first peak of the ATI spectrum is suppressed at 5×10⁻² W.cm⁻² with linear polarization and at a lower value in circular polarization. For both polarizations, the peak suppression takes place at intensities much lower than the saturation intensity. For circular polarization, the effect of the centrifugal barrier which has been proposed recently is discussed, with the conclusion that it may enhance the peak supression which remain, fundamentally an intensity effect.

© 1986 Optical Society of America