Faculty of Physics - University of Bucharest

UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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Conference: Bucharest University Faculty of Physics 2012 Meeting

Section: Theoretical Physics and Applied Mathematics

Title:

Retardation effects in two-photon ionization of ground state hydrogen-like atoms

Authors:

O. BUDRIGA(1), Viorica FLORESCU(2), H. BACHAU(3)

Affiliation:

1. National Institute for Laser, Plasma and Radiation Physics

2. Faculty of Physics, University of Bucharest

3. Centre des Lasers Intenses et Applications, Universite Bordeaux I-CNRS-CEA

E-mail

olimpia.budriga@inflpr.ro

Keywords:

Retardation effects, two-photon ionization, x-ray photon energy, hydrogen-like atoms

Abstract:

We investigate retardation effects in the ionization of ground state hydrogen-like atoms with low atomic number Z through the absorption of two identical photons. We use non-relativistic second-order perturbation theory which leads to an analytic expression of the transition amplitude in terms of Lauricella functions FD of four complex variables. We derive approximate formulas for the contribution of first retardation correction (quadrupole effects) which modify the electron angular distribution when compared with dipole approximation (DA). Our numerical investigation is done for photon energies between 0.2 keV and 25 keV. For linearly polarized photons, we describe the electron direction by its spherical angles in a system of reference with the 0z along the photon momentum and the 0x axis along the polarization. For Z=1 and photon energies below 2 keV the distributions in the polar angle at fixed azimuthal angle reveal the presence of the quadrupole contribution. In contrast to DA, the distributions are not symmetric around pi/2. When the electron is scattered in the plane orthogonal on the polarization direction, the sign of the relative difference between retardation included and DA results changes at pi/2, proving the effect of the quadrupole term. At higher energies the distributions indicate the presence of terms of second order in the photon momentum. We have considered also the total generalized cross sections, to which the quadrupole term do not contribute, in its dependence on photon energy and atomic number Z. Agreement was found in a comparison of our results with independent recent calculations based on the integration of time dependent Schrodinger equation.