Isotope effect in the dissociative attachment in H₂ at low energy

Abstract

The dissociative attachment cross section for formation of H^- and D^- from H₂, HD, and D₂ has been measured in the threshold range from 3.75 to 5 eV using a mass spectrometer for ion identification. A pronounced isotope effect is observed. The cross section of H^- from H₂ has a peak value of 1.6×10^-21 cm²; D^- from HD, 1×10^-22cm²; and D^- from D₂, 8×10^-24 cm²; all peaks occurring at 3.75 eV. Negative-ion formation in this energy range proceeds via the compound state ²∑_u⁺ whose lifetime against autodetachment is calculated from the above cross sections to be about 1×10^-15 sec. Because of this short lifetime and the relatively long time needed for the atoms to separate, the probability of survival of the negative ion, and therefore the dissociative-attachment cross section in H₂ at 3.75 eV, is small. For the heavier isotopes, the separation time is even longer and therefore the dissociative-attachment cross section is minute. The same compound state is also responsible for vibrational excitation of the hydrogen molecule.