No abstract

A second Born treatment is applied to study the (e, 3e) and (e, 3 − 1e) reactions for H2 targets. The results of this approximation are compared to the (e, 3 − 1e) experimental data obtained at about 600 eV impact energy. Several single-centre wavefunctions are used to describe the initial state and excited states of the molecule. Even if the second Born approximation is able to explain part of the experimental results the agreement is not good, similar to the case of the double ionization of helium (Götz et al 2003 J. Phys. B: At. Mol. Opt. Phys. 36 L77).

The second Born approximation is applied to study the (e, 2e) reaction for H2 targets. The results of this approximation are compared to a new set of experimental data obtained at about 600 eV impact energy, as well as to previous experiments performed at larger (∼4 keV) or smaller (250 eV) energy. Several single-centre wavefunctions are used to describe the initial state and excited states of the molecule. A generally good agreement is obtained with the experiments and with recent calculations.

We carry out a theoretical analysis of the double ionization of the water molecule by fast electrons. The analysis is based on a perturbative approach. For the final state we employ the well-known 3C wavefunction which has the correct asymptotic behaviour when all interparticle distances are large. The initial state of the target is described by an accurate molecular wavefunction proposed by Moccia (1964 J. Chem. Phys. A 40 2186). We present five-fold differential cross sections for a wide variety of kinematical conditions. We show that a simple summation over four indices is able to avoid a time-consuming triple numerical integration required to take account of the orientation of the molecule in space. The mechanisms of the double ionization of the water molecule are identified and discussed.

We apply the second Born approximation and the BBK methods to study triple-differential cross sections for the ionization of valence orbitals of a thymine molecule by electrons and positrons. Calculations have been performed for a coplanar geometry at an incident energy of 250 eV and an ejected-electron energy of 20 eV, while the angle of scattering is fixed at 10°. We use an accurate single-centre wavefunction for the initial state of the target and the well-known CNDO model. The present second Born approximation (with the single-centre wavefunction for the initial state) and the BBK model (with the CNDO model) yield cross sections in good agreement with the recent experimental data for electron impact. In the case of positron impact, we find that the contribution of the second term of the Born series is not insignificant for the present kinematics.