Double Ionization of Helium by 40 MeV Protons

Abstract

Differential ionization cross-sections for 40 MeV H + + He collisions were measured as a function of electron energy, electron ejection angle and final recoil ion charge state. Coincidences between recoil ions and emitted electrons were taken for electron ejection angles of 55" and 125" and energies between 3 and 500eV. Good agreement is found between measured energy distributions of ejected electrons and results of standard first-order perturbation theory in the case of single ionization. The experimental double-ionization cross-sections, however, are found to exceed independent-electron model predictions by more than an order of magnitude. Furthermore, evidence is provided that this deviation is mainly due to initial-state correlation.Introduction. -Double ionization of helium by ion impact has been studied extensively in the energy range 1-15 MeV/a.m.u. [l-31. In this energy range, the first Born approximation is valid for single ionization by proton impact, whereas it is not for double ionization as proved by the fact that the ratio between single-and double-ionization cross-sections depends on the sign of the projectile charge Zp. In other terms, the double-ionization cross-section does not vary as Zf for small 2, as the single-ionization cross-section does. The actual behaviour of the ratio between single-and double-ionization cross-sections has been discussed by many authors (for a review, see [4,5]). However, only one calculation reproduces correctly the difference between double ionization by protons and antiprotons [6]. Tentatively, various mechanisms have been suggested as listed, e.g., in [4]: ground-state correlation, TS-1, TS-2, shake-off, exchange contribution, scattering correlation. These concepts are often used without an unambiguous definition and may not be independent. Furthermore, interference between them or between first and second order of perturbation theory have been invoked. However, total cross-sections correspond to averages (over DAAD fellow.

Double Ionization of He by Fast Protons at Large Energy Transfer

“…This has indeed been observed. A substantially higher ratio near 2%, very close to that for photoionization, is observed in fast proton-He collisions, where the fast primary electrons were selected either kinematically [26] or directly [27,28].…”

“…This has indeed been observed. A substantially higher ratio near 2%, very close to that for photoionization, is observed in fast proton-He collisions, where the fast primary electrons were selected either kinematically [26] or directly [27,28].On the theoretical side, the subject has drawn renewed interest [2,29-31] very recently, partially driven by the flurry of new experiments at synchrotron light sources [10 -12,19], and theoretical calculations for photoionization and Compton scattering [13][14][15][16][17][18][20][21][22][23]. Although a higher ratio close to that for photoionization was observed for charged particles when fast electrons were isolated [26][27][28], the difference between charged particle scattering and photoionization still remains.…”

“…Here the energy transfer DE was measured directly and completely by measuring the energy loss of protons after the collision, whereas previously DE was either calculated by assuming binary kinematics between protons and electrons [26] or measured at one particular electron emission angle [27,28]. The new method has the advantage of much higher efficiency, equivalent to measuring electrons emitted into 4p solid angle, and thus avoids the possible complication due to angular correlation [28]. Indeed, it provides us the opportunity to measure R͑DE͒ at much larger DE, where a trend of decreasing R͑DE͒ with increasing DE has been observed for the first time.…”

“…Although a higher ratio close to that for photoionization was observed for charged particles when fast electrons were isolated [26][27][28], the difference between charged particle scattering and photoionization still remains. While a charged particle probes electrons in all its initial momentum space, a photon only interacts with electrons with high initial momentum in photoionization because the photon is annihilated and it carries very little momentum.…”

Differential Double Ionization of He by Compton Photons and Charged Particles at Large Energy Transfers

“…New synchrotron radiation sources have extended the photon energy range such that Compton scattering, i.e., inelastic scattering of photons, can dominate over photoionization [3][4][5][6][7][8][9]. At the same time, advances in experiments have been reported in which double ionization by fast charged particles is measured at large energy transfers, e, either directly [10] or indirectly [11][12][13]. At large e Compton scattering and charged particles probe the correlated two-electron initial state over the whole coordinate space, thus providing new and complementary information to photoionization that is sensitive to the localized region near the nucleus.…”