(e,2e) triple differential cross sections for the simultaneous ionization and excitation of helium

Abstract

The authors present absolute triple differential cross sections (TDCS) measurements for ionization of helium leaving the ion in both n=1 and n=2 final states, obtained under asymmetric geometry at an incident energy approximately 5.5 keV and ejected electron energies of 5, 10 and 75 eV. The kinematics are chosen to correspond either to a constant ejection energy, or to a constant energy transfer to the target. Angular distributions are measured at both constant ejection angle ( theta a mode) and at constant scattering angle ( theta b mode). In the theta a mode experiments, the momentum transfer dependence of the n=2 triple differential generalized oscillator strength is investigated for the first time. In both modes, the n=2 angular distributions show several new features which are not present for the n=1 ones, and which tend to vanish as the ejected energy is increased. They are attributed to final state interactions between the ejected electron and the excited ion. Comparison with first-order theoretical models shows the inadequacy of a Coulomb wave representation of the ejected electron, while in the R-matrix formalism it is found that a five-state multichannel calculation qualitatively describes the shape (but not the amplitude) of the TDCS measured in the theta b mode. Comparison is also made with the photoionization in the dipolar limit where the momentum transfer approaches zero. When integrated over the ejection direction, the double differential generalized oscillator strength ratio for ionization to the n=1 and n=2 states is found to agree with an earlier first Born close coupling prediction.

Theoretical triple differential cross section of the helium atom ionization with excitation to then=2 ion state

“…Various experimental techniques have been used including photon [1 -5], electron [6,7], and proton [8,9] impact. The relative contribution from the second-order and correlational processes was different in these studies.…”

“…The coincidence electron-impact ionization, or (e, 2e), experiments of Stefani, Avaldi, and Camilloni [6] and Dupre et al [7] can also be analyzed in the same terms of 'Electronic address: anatoli esm. ph.…”

“…au the relative contribution from the second-order and correlational processes, which is defined uniquely since the kinematics of all the particles involved in the reaction is fully determined. Such an analysis was attempted by Dupre et al [7], who tried to reproduce their own experimental results within a first-order model with the full account for both the ground-state and final-state correlations. They employed a highly correlated variational multiconfiguration wave function for the ground state while the multichannel R-matrix theory was used for the final state.…”

Electron-Impact Ionization and Excitation of Helium to then=1–4Ionic States

“…Even for the coplanar geometry, significant discrepancies between theory and experiment can occur under such conditions, if the part of the projectile-target interaction responsible for ionization is accounted for only to first order [12]. Such experiments were performed by Dupré et al [13] and later by Avaldi et al [14] and Rouvellou et al [15]. For their conditions of highly asymmetric energy sharing between the two outgoing electrons, hybrid methods based upon a perturbative treatment, up to second order, of the projectile-target interaction and an R matrix (closecoupling) approach for the ejected-electron-residual-ion interaction [16] were quite successful, especially when the latter part of the problem (e-He collisions) was systematically driven to convergence [17].…”

The (e,2e) spectroscopy, achievements and perspectives

“…The dynamical effects, that are typically observed in these measurements, as modification of the ratio between the intensities of the binary and recoil lobes or as angular shifts in the low energy electron distributions, are larger in the ionisation excitation process than in the pure ionisation. A similar study was performed by Dupre' et al at higher energy (46). In this latter experiment the absolute value of the cross section was also determined.…”