Ionizing collisions of long-lived highly excited hydrogen molecules with H2O and SF6

Long-lived highly excited hydrogen molecules (H2 * *) were produced by low-energy electron impact on H2, and allowed to collide with H2O and SF6. The resulting H2+ ions were observed by a quadrupole mass spectrometer. It was confirmed by the threshold energy, 15.4 +- 0.3 eV, and by field ionization that the precursor of H2+ was the H2 molecule in high Rydberg states, their principal quantum numbers being estimated to be between 20 and 40. The shape of the excitation function suggested that H2 * * molecules were mainly produced by optically forbidden transitions. The ionization cross sections of H2 * * and D2 * * with H2O were estimated by the dependence of the H2+ and D2+ intensities on the H2O pressure to be (3.8 +- 1.0) x 10-13 and (5.5 +- 1.5) x 10-13 cm2, respectively. The average kinetic energy of H2 * * was calculated by a TOF measurement to be 75 +- 10 meV. (author)