Double excitation of helium by fast particles of charge<i>Z</i>

Abstract

Results of calculations of double excitation of helium to n=2 states for fast projectiles of charge Z are presented. Nonzero ${\mathit{Z}}^{3}$ contributions for single and double excitation occur only when time-ordering contributions from the second-order amplitude are nonzero. For double excitation, electron correlation must also be nonzero to obtain ${\mathit{Z}}^{3}$ terms. The time-ordering effects arise from virtual off-energy-shell intermediate states. As with second-order amplitudes for Thomas singularities in electron capture, the energy-nonconserving amplitude is connected to the second-order energy-conserving amplitude by a dispersion relation. Comparison is made with experiment.