We propose a unified R-matrix-FLoquct method which can he used to analyse both the multiphoton ionization of a t o m and laser-assisted electron-atom collisions. Our treatment is nompertwhative and can be applied to an arbitrary LII"'II.
The R-matrix method has been applied to study the scattering of electrons by molecular oxygen for a number of internuclear separations, R, including the equilibrium separation of Re approximately=2.3a0. Results have been obtained for 12 scattering symmetries for impact energies ranging from threshold to 1.0 Ryd. Summed cross sections for elastic scattering from the ground state, and the corresponding cross sections for transitions from the ground state to the a 1 Delta g, b1 Sigma g+ and the '6 eV' states of oxygen are presented and are compared with experimental data. Finally, the position and width of the resonance in the 2 Pi g and 2 Pi u scattering symmetries are tabulated for values of R ranging from 1.85 to 2.90 a0.
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
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