Effects of deuteration on vibrational excitation in the products of the electron recombination of HCO+ and N2H+

Optical emissions from the CO (a3 Πr) and N2 (B3Πg) electronically excited products of the dissociative electron recombination of vibrationally relaxed HCO+ and N2H+ and their deuterated analogues have been detected from flowing afterglow plasmas. From the CO (a → X1 Σ+) and N2 (B→ A3 Σ+ u) emissions, vibrational population distributions were determined for the CO a (ν' = 0-6) and N2 B (ν' = 2-9) states. The changes in the vibrational populations with ν', and somewhat unexpectedly the changes in the overall distributions on deuteration, were smaller than suggested by the ‘impulse’ theory of Bates, which predicts a marked decrease with increasing ν' in both cases. For CO, the observed variations are more compatible with the Bates' ‘relic’ mechanism, if the restriction of zero overlap for the CO components of the wavefunction in the ion and the free diatomic is relaxed. For N2, in the change in the distribution on deuteration, a population change is seen around the vibrational level (ν' = 6) which is almost resonant with the energy of the recombining ion. Here the populations in the hydrogenated system are enhanced, suggesting the involvement of a tunnelling mechanism. The variations are qualitatively consistent with a simple theoretical model for tunnelling.