Direct above-threshold ionization in intense few-cycle laser pulses: structures in the angle-integrated energy spectra

This paper concerns the theory of non-recollisional ionization or detachment of atoms or ions by intense few-cycle pulses. It is shown that in certain conditions of pulse duration, peak intensity and carrier-envelope phase, the ionization probability integrated over ejection angle varies almost periodically with energy, with a period roughly equal to the photon energy for slow enough outgoing electrons. This modulation is found both in calculations based on the strong field approximation (SFA) and in ab initio time-dependent calculations. It is explained as resulting from the interference between the contributions of different saddle times of the modified classical action. Methods for efficiently calculating the SFA ionization amplitude beyond the usual saddle point approximation are also discussed.