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Electron impact has long been used1 to study the angular distribution of Auger electrons resulting from the electron rearrangement consecutive to an intermediate or inner shell ionization. In the case of an inner shell vacancy (nlj) with quantum number j>1/2, caused by impact ionization of a directed and unpolarized electron beam, the ion in general is left in a non-isotropic state. This anisotropy, or alignment of the ion, shows up in the anisotropy of the angular distribution of the decay products, namely the emitted x-ray photon or Auger electron.

The double ionization of He by electron impact at 580 eV has been studied in a coplanar symmetric (e,(3–1)e) experiment, in which two fast electrons of 250 eV are detected. In this way a momentum transfer as large as 6 a.u. is achieved. The results are compared with the predictions of a theoretical model based on the impulse (knock-out) approximation. The calculations, which include radial correlation of electrons in the He ground state wave function, better describe the experiment than those with an uncorrelated wave function.

We determine, both experimentally and theoretically, the fully resolved fivefold differential cross section (5DCS) of double ionization of helium by 5.6 keV electron impact. Symmetric energy sharing between the two ejected electrons is investigated at the excess energy of 8 and 20 eV with 0.22 and 0.24 au momentum transfer, respectively. Absolute 5DCS are determined by normalizing the experimental data to the well established single-ionization cross sections. The calculation is performed by using the convergent close-coupling method for the interaction between the two slow ejected electrons, together with the first Born approximation for the interaction of the fast incident electron with the atom. Experimental and theoretical 5DCS tend to agree in shape but disagree in magnitude by factors of three and 14 for the 20 and 8 eV excess energies, respectively. The small momentum transfer invites absolute comparison of the present electron-impact double-ionization results with the corresponding double-photoionization experiment and theory. Theoretically, the momentum transfer is sufficiently close to zero to show the scaling between the two scattering processes. This smallness of the momentum transfer also makes the calculated 5DCS nearly invariant with respect to simultaneous inversion of the momenta of the two ejected electrons.

The triple differential cross sections for electron impact ionisation of argon on its 3p orbital are investigated for a primary electron energy near 8 keV. In a coplanar coincidence experiment, both ionisation electrons are analysed with respect to their energy and scattering angles. As in previous low-energy measurements, two lobes are found in the angular distribution of the slow ejected electron. The data are made absolute to an accuracy of better than 10%. This is done at high energy and large momentum transfer, K, by normalising to the impulse approximation Compton profile and assuming a cylindrical symmetry of the data about K. At high momentum transfer where the collision may be considered as a binary electron-electron process, the Ar 3p electron momentum distribution is determined and found to be in very good agreement with the Fourier transform of the Hartree-Fock-Clementi wavefunction.