Direct and sequential ejection of the two valence electrons of beryllium by ultrashort laser pulses with photon energies below 18.2 eV

Abstract

We investigate the two-photon double ionization of a beryllium atom by intense ultrashort laser pulses. We apply a nonperturbative approach to solve the time-dependent Schrödinger equation. We treat the atom as a two-active-electron system with two outer electrons in the field of a frozen core that includes the nucleus and the electrons of the 1s2 inner shell. We trigger the ionization with photon energies lower than the second ionization potential Ip(Be+(2s)) = 18.2 eV of the atom so that the 2s2 valence shell electrons can be directly ejected in the double continuum or sequentially ejected via excitation ionization through the Be+(2p) ion. We investigate, for various photon energies, the contribution of each of the direct and sequential processes to the ionization dynamic.