Ejection of Quasi-Free-Electron Pairs from the Helium-Atom Ground State by Single-Photon Absorption

Abstract

We investigate single photon double ionization (PDI) of helium at photon energies of 440 and 800 eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back-to-back with equal energy. These slow ions are the unique fingerprint of an elusive quasi-free PDI mechanism predicted by Amusia et al. nearly four decades years ago [J. Phys. B 8, 1248Phys. B 8, , (1975] . It results from the non-dipole part of the electromagnetic interaction. Our experimental data are in excellent agreement with calculations performed using the convergent close coupling and time dependent close coupling methods.

Photon Momentum Transfer in Single-Photon Double Ionization of Helium

“…E 1 =c is the scaling one would expect if the photon momentum couples to the electron and not to the nucleus. This supports the intuitive explanation that the QFM is a process where the electron pair absorbs the photon while the nucleus remains a spectator [9][10][11] absorbing none of the photon's linear momentum, energy, or angular momentum. The QFM contributes only very close to equal energy sharing.…”

“…1, one observes that the nondipole enhancement is extremely obvious at low momenta, which becomes more and more noticeable with the increase in photon energy. Such a kind of nondipole-induced enhancement at low ion momentum has previously been explained with the quasifree mechanism (QFM) [10,11,34]. The QFM, which contributes to the pure-quadrupole yield, can be taken as a benchmark to examine the calculations.…”

“…However, many novel nondipole effects have recently been identified in experiments on single-photon double ionization (SPDI) of atoms. In particular, for SPDI of helium at extremely large photon energies, the breakdown of one dipole selection rule, i.e., the forbidding of back-to-back emission of electrons with an equal energy, has been found [9][10][11]. Furthermore, a forward-backward asymmetry of the photoelectron angular distribution along the light propagation direction, which was previously known for the single ionization [12,13], has also been predicted in the SPDI [14][15][16] and the two-photon double ionization (TPDI) [17,18].…”

“…Single-photon double ionization is frequently pictured as being composed of two steps: the interaction of the light field with one of the electrons and a subsequent step in which the second electron is set free, mediated by the PHYSICAL REVIEW LETTERS 124, 043201 (2020) 043201-2 electron-electron correlation as manifested in the shake-off [35][36][37], the knock-out [37], or the quasifree mechanism [10,11,34]. The forward-backward asymmetry of the ion along the light propagation is useful to check the validity of the dipole-quadrupole interference [12].…”

Agreement of Experiment and Theory on the Single Ionization of Helium by Fast Proton Impact

“…In these cases many delicate mechanisms of interaction can manifest themselves. These can be contribution from higher order terms in the Born series, with prominent examples being the turn-up effect in (e,2e) electron momentum spectroscopy of atoms [1], effects of the photon momentum in (γ,2e) [2,3] and the dipole Cooper minimum in photoionization [4,5], etc.…”