TWO-PHOTON TWO-ELECTRON IONIZATION OF HELIUM: A THEORETICAL APPROACH USING COULOMB-VOLKOV STATES

R. GUICHARD; H. BACHAU; R. GAYET; V. D. RODRÍGUEZ

Freiburg, Germany

Congreso; 25th International Conference on Photonic, electronic and Atomic Collisions; 2007

ICPEAC

A short, in some case intense, laser pulse interacting with matter can produce manifold processes involving the whole electronic spectrum of the element under study. In this framework, analytical approximate theories based on "Coulomb-Volkov" (CV) type states have shown their efficiency to describe multiphoton transitions induced by femtosecond laser pulses in single-electron systems, such as hydrogen, in various cases: in perturbation conditions both, when photon energy (hw) is greater than the ionization potential (Ip), and when hw < Ip , but also in non-perturbative conditions when hw > Ip . In the case of XUV laser fields interacting with many-electron atoms, multiphoton processes often involve more than one electron. Our current work is the implementation of CV theory to systems such as helium in order to probe electronic correlations effects. We focus here on two-photon two-electron ionization with photon energy such that double ionization is dominated by "direct" two-photons absorption path, i.e., 1.45 u.a. < w < 2 u.a. . We have restricted our calculations to this later case in order to compare with former predictions. Moreover, addressing "sequential" path through a CV theory is not an easy task . One expects to have access to exhaustive information such as angular and energetic distributions, cross-sections etc. A first simple model has been developed. It includes screening effects in initial state, but electron correlations in the final continuum state are ignored. Thus, the initial wave function is a simple product of hydrogen-like orbitals with an effective charge while the final one is an antisymmetrized product of continuum-like wave functions. First we have checked the validity of the CV approach by comparing the cross section obtained for two-photon two-electron ejection with previous results. Our analysis shows that CV well represents the process where both photons are simultaneously absorbed by the "1s" electrons and emitted to the double continuum. This double-ionization regime occurs when the photon energy is well below 2. a.u. (but above 1.45 a.u.). For photon energies approaching 2. a.u. double ionization involves intermediate singly ionized states, i.e., a path which is not represented in CV approach. Then we have tested various approximations to represent the electron correlations in initial and final states. In particular we have tested the function of Silverman et al for the initial state and a trial wave function that describes the electron moving asymptotically in Coulomb potential for the final double continuum. Total and differential two-electron ejection cross sections are discussed and compared with recents results.