Isotope Effect in the Dissociative Attachment in H2 at Low Energy

摘要

The dissociative attachment cross section for formation of H- and D- from H2, HD, and D2 was 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 x 10 to the -21 power sq. cm, D- from HD, 1 x 10 to the -22 power sq. cm, and D- from D2, 8 x 10 to the -24 power sq. cm, all peaks occurring at 3.75 eV. Negative ion formation in this energy range proceeds via the compound state, doublet-Sigma + sub u, whose lifetime against autodetachment is calculated from the above cross sections to be about 1 x 10 to the -15 power 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. (Author)