Complete momentum balance for single ionization of helium by fast ion impact: Experiment

Abstract

The collision dynamics of He single ionization by 3.6 MeV/u Se 28ϩ impact was explored using the reaction microscope of the Gesellschaft für Schwerionenforschung, a high-resolution integrated multielectron recoil-ion momentum spectrometer. The complete three-particle final-state momentum distribution ͑nine Cartesian components p i ͒ was imaged with a resolution of ⌬p i ϷϮ0.1 a.u. by measuring the three momentum components of the emitted electron and the recoiling target ion in coincidence. The projectile energy loss has been determined on a level of ⌬E p /E p Ϸ10 Ϫ7 and projectile scattering angles as small as ⌬Ϸ10 Ϫ7 rad became accessible. The experimental data which are compared with results of classical trajectory Monte Carlo calculations reveal an unprecedented insight into the details of the electron emission and the collision dynamics for ionization of helium by fast heavy-ion impact.

Intermediate energy ionization of helium by proton impact

“…This, however, is not a serious discrepancy when one considers that the theory stops at a finite time and shows results only for one impact parameter. The results for the longitudinal momentum distributions come as no big surprise since it is known that the ejected electron momentum distribution is target centred for high impact energies [24], but the time evolution of the longitudinal momentum distribution, discussed in the next section, shows an interesting evolution towards this expected result.…”

“…At these higher velocities it is no longer possible to identify a scattering plane unambiguously, because the momentum transfers between projectile and electron and between projectile and target nucleus are comparable, and because the use of classical trajectories becomes invalid in principle. Two major mechanisms for electron ejection have been identified, namely 'dipole-like' ionization involving momentum exchange between electron and target ion (recoil) and 'binary encounter' or 'two-body' ionization, involving momentum exchange between projectile and electron [24]. In the former process the projectile provides energy but not momentum, analogous to photoionization.…”

“…Non-coincident electron spectra from this process have been heavily studied for decades and treatments by various forms of perturbation theory have found considerable success [23]. Imaging data for ionization of He by fast protons have been taken by several authors [24][25][26][27]. At these higher velocities it is no longer possible to identify a scattering plane unambiguously, because the momentum transfers between projectile and electron and between projectile and target nucleus are comparable, and because the use of classical trajectories becomes invalid in principle.…”

Cold Target Recoil Ion Momentum Spectroscopy

“…single ionization by 3.6 MeV/u Se > and Ni > impact) Moshammer et al have shown that electrons and ions are still emitted mainly back to back [76,121,122,170]. Even though the equivalent photon method is generally not expected to be valid in this velocity regime, the experiments show that the projectile momentum exchange is much smaller than the recoil ion and electron momenta.…”

“…The GSI spectrometer has been used for kinematically complete measurements of the single and multiple ionization process in fast highly charged ion atom collisions [76,121,122] and (e,3e) studies [17]. A similar setup has been used for investigation of photo double ionization of He [123].…”