Separating Dipole and Quadrupole Contributions to Single-Photon Double Ionization

Abstract

We report on a kinematically complete measurement of double ionization of helium by a single 1100 eV circularly polarized photon. By exploiting dipole selection rules in the two-electron continuum state, we observed the angular emission pattern of electrons originating from a pure quadrupole transition. Our fully differential experimental data and companion ab initio nonperturbative theory show the separation of dipole and quadrupole contributions to photo-double-ionization and provide new insight into the nature of the quasifree mechanism.

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.…”

“…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].…”

“…In the present Letter, we address this question in SPDI by theoretically solving the fulldimensional time-dependent Schrödinger equation (TDSE) beyond the dipole approximation and comparing the results with experimental measurements carried out at beam line P04 (PETRA III, DESY, Hamburg). (See [11] for details on the experiment, where a subset of the results for photon energy of 1100 eV is presented.) The data for 385 and 800 eV are shown here for the first time.…”

“…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.…”

“…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].…”