MECHANISMS OF DOUBLE EXCITATION IN ION-ATOM COLLISIONS AT HIGH VELOCITIES

Double-excitation cross sections of helium atoms by bare heavy-ion projectiles at high energies are calculated within the one-center atomic-orbital expansion method including the electron-electron interaction between correlated helium wave functions. The resulting excitation cross sections to 2s2 1Se, 2p2 1De, 2s2p 1Po, and 2p2 1Se states are analyzed. We conclude that double excitations at high energies by low-Zp projectiles are dominated by a first-order process in which the two-electron transition proceeds through one electron-projectile interaction in conjunction with shakeup or rearrangement due to electron-electron interactions. For high-Zp projectiles at a given velocity, on the other hand, the excitation is dominated by a second-order process in which the transition proceeds through two successive electron-projectile interactions. © 1990 The American Physical Society.