In-depth analysis of Coulomb–Volkov approaches to ionization and excitation by laser pulses

Abstract

In perturbation conditions, above-threshold ionization spectra produced in the interaction of atoms with femtosecond short-wavelength laser pulses are well predicted by a theoretical approach called CV2^-, which is based on Coulomb–Volkov-type states. However, when resonant intermediate states play a significant role in a multiphoton transition, the CV2^- transition amplitude does not take their influence into account. In a previous paper, this influence has been introduced separately as a series of additional sequential processes interfering with the direct process. To give more credit to this procedure, called modified CV2^- (MCV2^-), a perturbation expansion of the standard CV2^- transition amplitude is compared here to the standard time-dependent perturbation series and the strong field approximation. It is shown that the CV2^- transition amplitude consists merely in a simultaneous absorption of all photons involved in the transition, thus avoiding all intermediate resonant state influence. The present analysis supports the MCV2^- procedure that consists in introducing explicitly the other quantum paths, which contribute significantly to ionization, such as passing through intermediate resonances. Further, this analysis permits to show that multiphoton excitation may be addressed by a Coulomb-Volkov approach akin to MCV2^-.