Ionizing Collisions of Long-Lived Highly Excited Hydrogen Molecules with H₂O and SF₆

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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)×10⁻¹³ and (5.5±1.5)×10⁻¹³ cm², respectively. The average kinetic energy of H₂^** was calculated by a TOF measurement to be 75±10 meV.