Effect of kinetic and electronic energy on the reactions of Cr+ with H2, HD, and D2

The reactions of atomic chromium ions with H2 , HD, and D2 are examined using guided ion beam tandem mass spectrometry. The ground electronic state (6S) is found to react inefficiently. The thresholds of these data are analyzed to yield a 0 K bond dissociation energy for CrH+ of 1.37 ± 0.09 eV (31.6 ± 2.0 kcal/mol). In the reaction of Cr+ (6S) with HD, formation of the metal deuteride ion is strongly favored over that of the metal hydride ion in the threshold region but not at higher energies. These results indicate that Cr+ (6S) reacts largely via an impulsive mechanism. No evidence for reaction of Cr+ (6Z>), the first excited state, is found but higher lying excited states, 4D and 4G (produced by electron impact), are found to react at their thermodynamic threshold. In reaction with HD, these states produce CrH+ preferentially by a factor of 2-4 over CrD+ . The differences in reactivity among these states can be explained by using simple molecular orbital concepts which have been developed for other atomic transition metal ions.