Double and single ionization of hydrogen molecules by fast-proton impact

The ratio of double to single ionization of hydrogen molecules caused by fast-proton impact was measured over a wide Velocity range(nu = 4-24 au) using the coincidence time-of-flight technique. The value of this ratio for hydrogen molecules at the high-velocity limit was determined to be 0.18(-0.02)(+0.01), Using the q/nu dependence suggested by McGuire. This ratio is smaller by about a factor of 1.8 for hydrogen than for helium over the measured energy range and by about 1.4 at the high-velocity limit. This difference between the two targets is due mainly to the single-ionization cross section, which was measured to be larger by a factor of 1.79 +/- 0.05 for hydrogen molecules than for helium. The double-ionization cross section, in contrast, is similar for both helium and hydrogen targets. It is suggested that single ionization of hydrogen molecules is more likely due to its smaller binding energy while the stronger electron-electron interaction in helium compensates for the smaller probability of proton impact ionization and leads to roughly equal double ionization of both targets. For both hydrogen and helium targets, the double- to single-ionization ratio is smaller for proton impact than for equal-velocity electron impact over the measured velocity range.