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

The ratio of double to single ionization of hydrogen molecules caused by fa st-proton impact was measured over a wide Velocity range(nu = 4-24 au) usin g the coincidence time-of-flight technique. The value of this ratio for hyd rogen 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 smal ler 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 sectio n, which was measured to be larger by a factor of 1.79 +/- 0.05 for hydroge n molecules than for helium. The double-ionization cross section, in contra st, is similar for both helium and hydrogen targets. It is suggested that s ingle ionization of hydrogen molecules is more likely due to its smaller bi nding energy while the stronger electron-electron interaction in helium com pensates for the smaller probability of proton impact ionization and leads to roughly equal double ionization of both targets. For both hydrogen and h elium targets, the double- to single-ionization ratio is smaller for proton impact than for equal-velocity electron impact over the measured velocity range.