Dissociative recombination of NO+: Dynamics of the X 1Σ+ and a 3Σ+ electronic states

Abstract

We have studied the dissociation dynamics of NO+ ions in their ground, X 1Σ+, and first excited metastable, a 3Σ+ states, induced by the capture of electrons of variable collision energy in the dissociative recombination (DR) process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO+ (X 1Σ+,v=0) ions dissociate dominantly to the N(2D)+O(3P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N(4S)+O(1D) dissociation limit contributes 0(±2)% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation during the dissociation and the initial electron capture. We also report the distribution of fragment pairs from the DR reaction involving the metastable a 3Σ+ state. This state is found to dissociate to nearly all of the energetically allowed product pairs. The lifetime of the a 3Σ+ state is found to be 730(±50) ms, in agreement with earlier, sometimes indirect, observations. The experimental observations have been complemented with ab initio calculations on the different radiative decay processes both for the X 1Σ+ and the a 3Σ+ states. It is found that vibrational relaxation via infrared radiation is faster for NO+ (a 3Σ+,v>0) ions than the electronic decay of these metastable-state ions to the ground state.