Experimental ionization cross sections for fast hydrogen atoms and protons in a water gas as a function of impact speed. “AR” refers to the additive rule: when no experimental results for H₂O are available, results for H₂ and O₂ are combined to give an estimate (see text for details); experimental uncertainties for these estimates are at least 25%. No measurement for the ionization of H₂O by H⁻ is available. Polynomial fits in thick lines are also shown, and the coefficients are listed in Table 4. Smooth extrapolations in power laws at low and high energies are indicated as thin dotted lines.