Intensity, pulse width, and polarization dependence of above-threshold-ionization electron spectra

Some difficulties in above-threshold-ionization experiments (mainly the intensity measurements and space-charge effects) are discussed. New results obtained in a space-charge-free regime are reported. The intensity dependences of the spectra show a clear departure from the predictions of lowest-order perturbation theory. At short pulse duration (50 psec) the peaks appear red shifted, and the effect is amplified in circular polarization. A possible interpretation of this effect in terms of ponderomotive potentials is proposed.