Non-linear excitation of atoms by ultrashort electromagnetic pulses

The development of methods of generation of ultrashort pulses (USP) of femto- and attosecond duration ranges with controlled parameters necessitates the theoretical study of features of their interaction with a matter [1]. Among such features that do not exist in case of "long" pulses should first of all be the nonlinear dependence of the photoprocess probability W on the USP duration (tau) [2] as well as the dependence on the carrier phase with respect to the pulse envelope (phi) [3-4]. It should be noted that if the dependence of the probability W on the phase phi manifests itself either only for very short pulses, when omega tau <1 (omega is carrier frequency of the pulse), or in case of a nonlinear photoprocess [3], the function W(t) can differ from a linear function in the limit omega tau > 1 too for fields of moderate strength, when the perturbation theory is applicable [2].