I will discuss our recent achievements in analytical description of above threshold ionization of an atomic system by a few-cycle laser pulse [1]. Our analytical approach is based on the alternative consideration of interaction of an atomic electron with a short pulse as a limiting case of interaction with a train of pulses separated in time by a period of T. For a finite T, interaction with the pulse train can be efficiently consider in the framework of quasistationary quasienergy states (QQES) approach, in which a self-consistent definition of detachment/ionization amplitude follows from the property of QQES wave function. Once the pulse train problem is solved, the result for a short pulse follows as the limit T→ ∞. Our analytical result involves physically transparent quantities (such as the decay rates in a static electric field and the cross section for elastic electron scattering from a short-range potential), we generalize our results to describe ATI of a neutral atom by replacing corresponding quantities by their atomic counterparts. To check the accuracy of our analytic results for ATI in a short pulse, we compare them with the results obtained from numerical solution of the time-dependent Schr?dinger equation for different atoms. Our results describe perfectly both the CEP-induced “back-forward” asymmetry in the high-energy part of the momentum distributions and the interference patterns originating from the interference of “short” and “long” quantum orbits. I will also discuss the factorization of the high-energy ATI probability in terms of the elastic scattering cross section and the CEP-dependent electronic wave packet W(E) and provide explicit form of W(E) in a few-cycle pulse.

[1] M.V. Frolov, D.V. Knyazeva, N.L. Manakov, A.M. Popov, O.V. Tikhonova, E.A. Volkova, Ming-Hui Xu, Liang-You Peng, Liang-Wen Pi, A.F. Starace, Phys. Rev. Lett. 108, 213002 (2012).