Exchange effects in strong-field ionization of the excited lithium ion induced by one- and two-component laser fields

Abstract

Spin-dependent effects in strong-laser-field-induced above-threshold ionization of excited Li+ ions, caused by the requirement that the electron wave function is antisymmetric, are analyzed using the strong-field approximation and saddle-point method. For an excited Li+ ion exposed to a linearly polarized laser field, the minima in the photoelectron momentum distribution in the polarization plane appear if the excited Li+ ion state is the spin singlet state, while for the spin triplet state these minima are absent. The difference between the spectra obtained with these spin states is quantified by the corresponding normalized difference of the differential ionization probabilities. Employing the saddle-point method, we show that, for the spin singlet state, all relevant contributions to the differential ionization probability exhibit minima for approximately the same values of the photoelectron energy and emission angle, thus leading to the minima in the total spectra. Similar conclusions hold for a bicircular driving field. In this case, the range of values of the photoelectron energy and emission angle for which different saddle-point contributions exhibit minima is almost the same for all saddle-point solutions. This is particularly true for the high-energy part of the spectrum, and the minima are more pronounced than for the linearly polarized driving field case. In order to check whether these minima can be detected in an experiment, we perform focal averaging, which takes into account the intensity distribution in the laser focus. For both linearly polarized and bicircular driving fields, the minima survive the focal-averaging procedure. They are slightly blurred in the former case, while in the latter case the focal averaging almost does not affect the minima at all. Finally, we confirm that similar conclusions hold for the photoelectron velocity maps in the plane that contains the laser-field propagation direction.