Solution of the many-electron many-photon problem for strong fields: Application toLi−in one- and two-color laser fields

Abstract

The solution of the many-electron many-photon (MEMP) problem for strong fields is facilitated if the corresponding theory entails a computational methodology that combines economy with accuracy and generality, as regards electronic structure and the incorporation of the continuous spectrum. By applying the nonperturbative MEMP theory (MEMPT) to the prototypical ${\mathrm{Li}}^{\ensuremath{-}}{}^{1}S$ state, where both radial and angular correlations in the initial state and interchannel couplings in the final scattering states cannot be ignored, we computed frequency-dependent widths \ensuremath{\Gamma}(\ensuremath{\omega}) of multiphoton detachment, as well as energy shifts \ensuremath{\Delta}(\ensuremath{\omega}), for intensities $1\ifmmode\times\else\texttimes\fi{}{10}^{9}--1\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},$ using one- as well as two-color fields. Even though the ${1s}^{2}2p{}^{2}{P}^{o}$ threshold is kept energetically closed, its coupling to the open channel ${1s}^{2}2s{}^{2}S$ cannot be ignored. For the two-color MEMP problem, the present application of the MEMPT provides results for a four-electron system, whereby the self-consistent field, electron correlation, and interchannel coupling are taken into account. The results for (\ensuremath{\omega}, 3\ensuremath{\omega}) laser fields exhibit the recently predicted [Th. Mercouris and C.A. Nicolaides, Phys. Rev. A 63, 013411 (2001)] linear dependence of the rate on $\mathrm{cos}\ensuremath{\varphi},$ where \ensuremath{\varphi} is the phase difference of the two weak fields. Based on this and on lowest-order perturbation theory (LOPT), we obtain a quantity characteristic of the system atom plus fields, which we name the ``interference generalized cross section.'' For the one-color system, comparison is made with our previous conclusions [C.A. Nicolaides and Th. Mercouris, Chem. Phys. Lett. 159, 45 (1989); J. Opt. Soc. Am. B 7, 494 (1990)] and with results from recent calculations of the two- and three-photon detachment rates by Glass et al. [J. Phys. B 31, L667 (1998)], who implemented R-matrix Floquet theory, and by Telnov and Chu [Phys. Rev. A 66, 043417 (2002)], who implemented time-dependent density-functional theory in the Floquet formulation via exterior complex scaling. Similarities as well as discrepancies are observed. Our results for \ensuremath{\Gamma}(\ensuremath{\omega}) and \ensuremath{\Delta}(\ensuremath{\omega}) involve a dense set of values as a function of \ensuremath{\omega} and provide a clear picture of the physics below, at, and above the $\stackrel{\ensuremath{\rightarrow}}{3}2$ photon threshold.