Absolute triple differential cross sections for electron impact ionisation of He at 1024.6 eV incident energy

First-order models have been extensively applied to the study of the interaction mechanism in electron impact ionisation. While first Born theories have been successfully applied in the dipolar regime (small momentum transfer), the impulse approximation has been used in the interpretation of (e, 2e) triple differential cross sections in the binary regime (high momentum transfer and symmetric geometry). This work investigates the intermediate regime where neither the simple first Born nor the impulsive approximation is correct. For this purpose (e, 2e) cross sections of He have been measured in asymmetric conditions at 1024.6 eV incident energy, for ejected electron energies ranging from 4 to 50 eV. Depending on the scattering angle, the momentum transfer K varied from 0.5 to 2 au. The experimental results have been compared with the theoretical predictions of the plane-wave impulse approximation (PWIA), the distorted-wave impulse approximation (DWIA) and the first Born approximation including an approximate post-collision interaction correction (BA-PCI). The results can be summarised as follows. (i) At low ejected electron energy correlation effects due to long-range Coulomb interactions are important. The BA-PCI calculation, which takes into account these effects, reproduces both the absolute value and the angular dependence of the experimental data at the smallest momentum transfer. (ii) Provided the kinematical conditions are confined to the Bethe ridge, where the recoil lobe becomes negligible, the cross section is well described by impulsive models even for momentum transfers as low as 2 au. The last result is of particular interest because it allows asymmetric conditions to be used for spectroscopic purposes