Two-photon double ionization of atomic beryllium with ultrashort laser pulses

Abstract

We investigate the two-photon double ionization of beryllium atom induced by ultrashort pulses. We use a time-dependent formalism to evaluate the ionization amplitudes and generalized cross sections for the ejection of the 2s 2 valence shell electrons in the presence of a fully occupied 1s 2 frozen core shell. The relative contributions of the two-photon direct and sequential process are systematically explored by varying both pulse duration and central frequency. The energy and angular differential ionization yields reveal the signatures of both mechanisms, as well as the role of electron correlation in both the single and double ionization continua. In contrast with previous results on the helium atom, the presence of an electronic core strongly affects the final state leading to back-to-back electron emission even in the a priori less correlated two-photon sequential mechanism. In particular, a dominant pathway via excitation ionization through the Be + (2p) determines the profiles and pulse-duration dependencies of the energy and angle differential yields.

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

“…In the Seq(A) process, the two valence electrons are ejected with close kinetic energies (E int < 1 eV) and are hence highly correlated. In contrast with the Seq(A) process via Be + (2p), the sequential ionization process Seq(B) that proceeds via the Be + (2s) (allowed only forhω > 18.2 eV) releases electrons that have distinct kinetic energies and are thus lowly correlated in energy sharing [5,12]. In the direct two-electron ejection process, the two photons of 15 eV each are absorbed almost simultaneously by the electron pair.…”

“…The frozen core approximation is valid only if the atom is submitted to photons that have just enough energy to eject the valence electrons, but not enough to reach the core avoiding hence the photoionization of the inner electrons. Few theoretical studies of the double ionization process by two-photon absorption in Be have been reported [5,12].…”

“…Both experiment and theory have investigated the single and double photoionization of two valence electrons atoms beginning with He to other few alkalineearth-metal atoms like Be or Mg [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. In the two-photon double ionization process, multiple pathways can be open for the electrons to reach the double continuum.…”

Fully differential single-photon double photoionization of atomic magnesium

“…The methods utilized in this work for treating photoionization of two electrons from a target with additional electrons has been previously described in greater detail in the both a time-independent framework, [8,9] and with a time-dependent treatment involving ultrashort laser pulses [11,21]. Thus, here we provide a brief overview of the most important points in applying the method to magnesium.…”

“…The formalism we employ to study this process requires denoting the neon-like core occupancy of all but the valence electrons, which provide a closed-shell Coulomb and exchange interaction with the outer 3s 2 electrons that feel the action of the photon towards the double continuum. The construction of atomic orbitals out of an underlying radial grid to represent these core electrons facilitates the approximation of holding them fixed in a configuration-interaction expansion and has been applied to atomic beryllium in both a time-independent formalism for one-photon double ionization [8,9,13], and in a time-dependent framework [11,21] for consideration of two-photon processes that remove the outer electrons. This method has also been applied to examine the double photoionization process for neon [22], which unlike the systems considered here, leaves behind an open-shell target with distinct final state couplings for the residual dication.…”