title: Ejection of quasi-free-electron pairs from the helium-atom ground state by single-photon absorption.
abstract: We investigate the single-photon double ionization 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 quasifree photon double ionization mechanism predicted by Amusia et al. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.
authors.affiliation: Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. schoeffler@atom.uni-frankfurt.de
authors.forename: M S
authors.initials: MS
authors.lastname: Schöffler
authors.forename: C
authors.initials: C
authors.lastname: Stuck
authors.forename: M
authors.initials: M
authors.lastname: Waitz
authors.forename: F
authors.initials: F
authors.lastname: Trinter
authors.forename: T
authors.initials: T
authors.lastname: Jahnke
authors.forename: U
authors.initials: U
authors.lastname: Lenz
authors.forename: M
authors.initials: M
authors.lastname: Jones
authors.forename: A
authors.initials: A
authors.lastname: Belkacem
authors.forename: A L
authors.initials: AL
authors.lastname: Landers
authors.forename: M S
authors.initials: MS
authors.lastname: Pindzola
authors.forename: C L
authors.initials: CL
authors.lastname: Cocke
authors.forename: J
authors.initials: J
authors.lastname: Colgan
authors.forename: A
authors.initials: A
authors.lastname: Kheifets
authors.forename: I
authors.initials: I
authors.lastname: Bray
authors.forename: H
authors.initials: H
authors.lastname: Schmidt-Böcking
authors.forename: R
authors.initials: R
authors.lastname: Dörner
authors.forename: Th
authors.initials: T
authors.lastname: Weber