ABSOLUTE CROSS-SECTIONS FOR HELIUM SINGLE AND DOUBLE IONIZATION IN COLLISIONS WITH FAST, HIGHLY CHARGED PROJECTILES

Absolute cross sections for single and double ionization of helium have been measured for highly charged (24 less-than-or-equal-to q less-than-or-equal-to 92) and fast (3.6 MeV u-1 less-than-or-equal-to E(p) less-than-or-equal-to 1 GeV u-1) heavy-ion impact. The ratios of double-to-single ionization are found to deviate drastically from the prediction of first-order theories: in the strong perturbation regime they fall below the scaling of Knudsen et al even for 120 MeV u-1 U91+ impact, whereas a strong increase by a factor of 3 relative to this semi-empirical prediction is observed in the regime of low perturbations for 0.5 and 1 GeV u-1 Kr36+ impact. At low reduced energies of E(p)/q less-than-or-similar-to 0.5 the single ionization cross sections are in excellent agreement with recent CDW-EIS calculations by Fainstein et al and close coupling calculations by Janev and Presnyakow. In the entire energy range the experimental data are predicted within 50% by classical calculations of McKenzie et al. For 3.6 MeV u-1 and projectile charge states 24 less-than-or-equal-to q less-than-or-equal-to 55 the cross sections scale slightly weaker than q2, as has been observed previously for the regime of strong coupling.