Double ionization of helium by fast bare ion collisions

A time-dependent close-coupling method is developed to treat the double ionization of helium by fast bare ion collisions. At high incident energies, charge transfer to the projectile is quite small, so that the two-electron wavefunction remains centred on the target, subject to a time-dependent projectile interaction. A multipole expansion of the projectile-atom interaction includes monopole, dipole, quadrupole and octopole terms. Time-dependent close-coupling calculations are carried out for alpha + He collisions at incident energies greater or equal to 1.0 MeV amu(-1). Over 100 coupled channels are needed to obtain total double ionization cross sections that are in good agreement with recent non-perturbative basis-set coupled channels calculations and absolute experimental measurements.