Ion-induced differential ionisation of helium at intermediate energies

Theoretical description of electron emission in ion collisions with multielectron targets remains a challenging problem. Even for the simplest case of bare ion scattering on helium, there are no nonperturbative approaches that can provide reliable information about all interconnected reaction channels, in particular, the kinematically complete differential picture of ionisation when electron capture to the continuum (ECC) is important. We have developed a wave-packet convergent close-coupling approach that is capable of providing benchmark data on the integrated and differential cross sections for all processes taking place in such collisions. The approach allows to calculate integrated, fully differential, as well as various doubly and singly differential cross sections for ionisation of atomic targets. The approach has recently been applied to p-He collisions. Calculations of ionisation cross sections differential in the electron emission energy, in the emission angle, as well as in the scattered-projectile angle have been performed in the intermediate energy region. The results agree well with experimental data where available. Moreover, our calculations reveal an interesting interplay between direct ionisation and ECC. The ionisation cross section differential in the angle of the ejected electron is dominated by ECC for ejection into small angles, while ejection into large angles is purely due to direct ionisation. The energy-differential cross section shows that low-energy electrons are emitted due to direct ionisation, while ECC leads to high-energy electrons. When ECC peaks, we see a shoulder in the cross section predicted by experiment. As to the singly differential cross section as a function of the scattered-projectile angle is concerned, at low projectile energies, direct ionisation and ECC have similar behaviour and comparable contributions. However, as the energy increases, direct ionisation becomes dominant.