The ratio $R$ of helium double-to-single ionization was determined with uncertainties below $1\%$ for 25, 120, and $225\mathrm{MeV}/\mathrm{u} {\mathrm{Fe}}^{26+}$ and $5\mathrm{MeV}/\mathrm{u} {\mathrm{O}}^{7+}$. The experimental results were compared with present theoretical calculations based on the semiclassical approximation and the independent electron model. Combined with the available historical data for highly charged projectiles, the results were analyzed in reduced-velocity representations. The present data lie within the perturbative two-step regime and fit well with the semiempirical reduced-velocity scaling. Reappraisal of the historical data further indicates that the previously reported deviations do not provide a consistent experimental basis for a breakdown of reduced-velocity scaling. For the ratio $R$ within the perturbative two-step regime, the available data therefore do not constitute robust experimental evidence for higher-order contributions.

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• Atomic & molecular collisions
• Electronic excitation & ionization