Frequency-domain theory of high-order above-threshold ionization based on nonperturbative quantum electrodynamics

High-order above-threshold ionization (ATI) is investigated in the frequency domain, based on a nonperturbative quantum electrodynamics theoretical approach. The transition matrix element of high-order ATI is expressed as a superposition of products of generalized Bessel functions, which represent probability amplitudes of finding electrons with given energies. From the frequency-domain viewpoint the high-order ATI can be described simply as an ATI followed by laser-assisted collision (LAC), and the features of high-order ATI reflect mainly the characteristics of LAC. We investigate thoroughly the LAC, finding that the plateau can be simulated by a simple classical model. We also discuss the correspondence between the time- and frequency-domain pictures of rescattering ATI