Time-dependentR-matrix theory of multiphoton processes

Abstract

A new time-dependent R-matrix theory of multiphoton processes is described which can be applied to an arbitrary many-electron atom. The theory is complementary to the R-matrix Floquet theory developed by Burke, Francken and Joachain (1991 J. Phys. B: At. Mol. Opt. Phys. 24 761) enabling processes involving higher laser field intensities and shorter laser pulses to be treated. The new theory is illustrated by analysing the multiphoton ionization of a charged particle bound initially in a one-dimensional potential well where the results are compared with an independent R-matrix Floquet calculation.In recent years there has been a considerable increase in interest in the study of the interaction of intense laser fields with many-electron atoms. Work in this area has been stimulated by increasingly powerful lasers which have led to the discovery of new phenomena such as high harmonic generation (e.g. L'Huillier et al 1992), above-threshold ionization (e.g. Muller et al 1992) and stabilization in super-intense high-frequency fields (Pont and Gavrila 1990). Nevertheless, in spite of this rapidly growing interest, most theoretical work until recently has been limited to atomic hydrogen or to single active electron models, where one electron is treated explicitly and the effect of the remaining electrons in the atom is treated in some average way (e.g. Potvliege and Shakeshaft 1988a, 1989.However, this situation has now begun to change as new theoretical approaches and computational methods are developed and as experimental interest moves increasingly to situations where electron-electron interaction effects in many-electron atoms play an important role. For example, in the case of two-electron atoms a major new programme of research has been initiated by Parker et al (1996) to solve the time-dependent twoelectron Schrödinger equation by direct numerical integration using a massively parallel supercomputer. In addition, a unified R-matrix Floquet theory has been developed by Burke, Francken and Joachain (1991), referred to hereafter as BFJ, which can be used to study both multiphoton ionization and laser-assisted electron-atom scattering for an arbitrary many-electron atom. Based on this theory, a general computer program package has been developed that has enabled multiphoton ionization and detachment rates as well as harmonic generation rates to be calculated for a number of complex atomic targets (e.g. see the recent review by Dörr 1997).The objective of the present letter is to present a new theory which will enable the timedependent Schrödinger equation for an arbitrary many-electron atom in an intense laser field to be solved directly using the R-matrix method. This work is complementary to the Rmatrix Floquet work mentioned above in that it will enable higher laser field intensities

Multiconfigurational Hartree-Fock close-coupling ansatz: Application to the argon photoionization cross section and delays

“…Only a limited number of such calculations exist today (see, for example, [62,[65][66][67][68]), all of them performed within the time-dependent R-matrix framework [67,69,70]. As a next step we will combine the present approach with a solution of the time-dependent Schrödinger equation along the lines of Refs.…”

Lippmann-Schwinger description of multiphoton ionization

“…This, however, is to be expected as for such strong fields the (-4,4) calculation is probably too small and a larger number of photons should be included to obtain more accurate results. Results in the Table I should be compared with the plot illustrating the level shift as a function of the electric field strength from the time-dependent R-matrix calculation of Burke and Burke [42]. Although the numerical values are not reported in this paper, a visual comparison of our data with the the plot quite satisfactory.…”

“…A more precise value of the field for which the channel corresponding to absorption of the three photons closes is F ≈ 0.1435 a.u. as shown by Burke and Burke [42]. Rich variety of phenomena may occur when field approaches this critical value (and larger ones, corresponding to closing of other channels), in particular ionization rates as functions of electric field intensity may have discontinuities [42].…”

Liouville-spaceR-matrix-Floquet description of atomic radiative processes involving autoionizing states in the presence of intense electromagnetic fields

“…The € "dressed" atomic states thereby obtained can then be used for the systematic investigation of an extensive class of atomic collision and radiation processes in the presence of intense, stationary (continuous-wave) electromagnetic fields. For a description of atomic processes in ultrashort-pulse electromagnetic fields, an alternative time-dependent R-matrix method has been developed (Burke and Burke 1997, van der Hart et al 2007, Lysaght et al 2008, and Guan et al 2007, 2008. A comprehensive discussion of the R-matrix-Floquet and the time-dependent R-matrix methods has been presented in a recent book by Burke (2011).…”