Dissociative recombination of H₂ ⁺: Product state information and very large cross sections of vibrationally excited H₂ ⁺

We report experiments on dissociative recombination (DR) of HD⁺ and H₂ ⁺. Product state information has been obtained over a wide range of electron energies with a position sensitive detector consisting of a graded absorber in combination with a surface-barrier detector. At low electron energies (<3 eV) hydrogen atoms are formed preferentially in highly excited states (n>2); at high electron energies (>12 eV) both hydrogen fragments are excited. The dissociative recombination rate of H₂ ⁺ has been measured also as a function of storage time in the energy range of 0 eV to 20 eV. We show that the H₂⁺ beam is still vibrationally excited after 20 s at our experimental conditions. The H₂ ⁺ ions relax vibrationally through interaction with electrons in the electron cooler. Vibrationally excited levels (v≥5) have DR rates that exceed the DR rate of the lower vibrational levels by two orders of magnitude. The latter observation has important consequences for the interpretation of previous DR experiments on H₂ ⁺. © 1996 The American Physical Society.