UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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Conference: Bucharest University Faculty of Physics 2011 Meeting


Section: Theoretical Physics and Applied Mathematics


Title:
Two-photon ionization of hydrogen K-shell from the photoelectric threshold up to 50 keV


Authors:
O. Budriga, Viorica Florescu


Affiliation:
1- National Institute for Laser, Plasma and Radiation Physics,Bucharest-Magurele 077125, Romania

2- Department of Physics and Centre for Advanced Quantum Physics,University of Bucharest, MG-11, Bucharest-Magurele, 077125 Romania


E-mail
olimpia.budriga@inflpr.ro


Keywords:
two-photon ionization,xuv and x radiation, retardation effects, relativistic effects


Abstract:
The new development of the x-ray free electron lasers in the domain of xuv and x photon energy range is the motivation for the study of the ionization of atoms through the simultaneous absorption of two photons in this laser energy range. The produced radiation is relatively not too intense, so the description based on perturbative methods is considered to be adequate. With the increase of the photon energy, the validity of the non-relativistic dipole approximation becomes questionable. We base our calculation on a hybrid approach in which the A2 contribution, which vanishes in the dipole approximation, is added to the AP second-order contribution. We use existing nonrelativistic analytic expressions for the two-photon ground state to continuum matrix element. The evaluation of the Appell functions was done using their standard integral representation. We have investigated several photon energies: i) from photoelectric threshold (13.6 eV) up to 1keV, ii) from 1 keV to 10 keV and iii) above 10keV up to 50 keV. We find complete agreement with published data, which cover energies up to 620 eV. In the first two regions DA gives the dominant contribution. The third region requires more investigation: retardation and relativistic effects begin to be visible. We have seen the influence of the pole present in the A2 term starting with 30 keV and we have checked that the pole disappears if the relativistic energy conservation law is used.