Triple Differential Cross Section Measurements for Double Photoionization ofD2

Abstract

Angular distributions of the two ejected electrons following photodouble ionization of molecular deuterium have been measured using a toroidal photoelectron-photoelectron coincidence spectrometer in conjunction with synchrotron radiation. Six ͑g, 2e͒ triple differential cross sections (TDCS) were measured in the plane orthogonal to the photon beam direction with E 1 E 2 10 eV. The angular distributions are similar to those of helium, but with differences which we highlight by comparing the D 2 TDCS with helium TDCS measured under nearly identical conditions. [S0031-9007(97)04143-4] PACS numbers: 33.80.Eh, 32.80.Fb In photodouble ionization (PDI) a single photon is absorbed by an atom or a molecule, followed by the ejection of two electrons. PDI is simplest for the fundamental two-electron systems, helium and molecular hydrogen, as there is only one possible ion state and there are no intermediate states above it:It is not possible for direct PDI to be described within the familiar independent particle model as the photon cannot interact with more than one electron. Consequently, PDI is a manifestation of electron-electron interactions and so is a sensitive test for electron correlation theories.The angular distribution-or triple differential cross section (TDCS)-of the ejected electrons is a particularly sensitive measure of the electron correlation. These measurements [e.g., 1-6] have only been possible during the last few years due to improvements in synchrotron radiation sources and in the necessary photoelectronphotoelectron coincidence (PEPECO) techniques. Recently, a complementary technique has been applied to helium which determines the momentum of the recoiling ion in coincidence with one of the photoelectrons, providing similar information but in momentum space [7]. To date, TDCS has been investigated only for rare gases, and the progress in this area has benefited greatly from a close relationship between experiment and theory (e.g., [8][9][10][11][12][13][14][15][16]). However, such studies have yet to be applied to molecules. In this Letter we present the first measurements of molecular TDCS, having performed them in the most fundamental system: hydrogen.The photodouble ionization of helium results in the classic three-particle Coulomb continuum problem, whose solution determines the correlation factor in the expression for the TDCS [8]. The situation in hydrogen is not unrelated, but is further complicated by the inevitable dissociation of the molecule during double ionization. Therefore, PDI in hydrogen results in four unbound particles, albeit with two types of particles having different charges, masses, and velocities. However, it is perhaps not unreasonable to expect similarities in the angular distributions with helium (final state electronic symmetry 1 P 0 ) if the experimental conditions are chosen such that the electrons' speeds are much greater than those of the protons. The nuclear motion could then be considered as a perturbation which increases as the electron energies decrease.Previou...

Photo double ionization of molecular deuterium

“…As seen in figure 4, this prediction is in good agreement with earlier data, which were taken with S 1 = 0.67 ± 0.03. This supporting evidence for our Reddish et al (1997a). The solid and dotted curves are the molecular and atomic forms of the TDCS, respectively, with the same 76 • correlation-function half-width.…”

“…• , an angle where the TDCS had been measured earlier (Reddish et al 1997a). The comparison of the TDCS, using both atomic and molecular descriptions but with the same correlation function half-widths θ 1/2 = 76…”

Recoil-ion and electron momentum spectroscopy: reaction-microscopes

“…One open problem is electron correlation in direct double ionisation of fixed in space H 2 (Kossmann et al 1989;Reddish et al 1997;Dörner et al 1998a;Reddish and Feagin 1998;Feagin 1998;Walter and Briggs 1999;Walter and Briggs 2000). In this case not only the internuclear axis but also the internuclear distance at the instant of photoabsorption can be measured via the ion fragment momenta.…”

Time-dependent calculations of double photoionization of the aligned H2molecule

“…In these kinematically complete, four-body fragmentation experiments, a strong dependence of the photoelectron angular distribution on the orientation of the molecular axis was demonstrated. This observation was not possible in earlier measurements on randomly oriented H 2 or D 2 molecules [4][5][6][7], which could be described rather adequately by atomiclike analytical models [8,9] or central-field numerical computations [10,11]. The new set of molecular-orientation-specific DPI measurements gave an impetus to the rapid advancement of theory.…”