Radiative decay and radiationless relaxation of NH/ND(a 1Δ) isolated in rare gas matrices

Abstract

Lifetimes of matrix-isolated NH/ND(a 1Δ) radicals have been measured as function of temperature and rare gas host. The metastable species were generated directly by in situ photolysis of hydrazoic or isocyanic acid, or by pulsed excitation of the b 1Σ+ state with a dye laser, which subsequently decays to the a state on a μs time scale. Rotation of NH/ND in the electronic ground state is perturbed or inhibited by the second photofragment in the photolysis systems. The lifetimes of NH(a 1Δ) in Ne, Ar, and Kr show little temperature dependence. This is consistent with a radiationless contribution to the overall relaxation in which the energy gap to the next lower vibrational level of the ground state is accepted by guest rotation and/or other local modes. The strong temperature dependent decay of ND(a 1Δ) in Ar and Kr is due to endothermic near resonant relaxation to the sixth vibrational level of the ground state, with an activation energy in the order of the energy gap, and a frequency factor of 20±5 s−1. The relaxation mechanism of NH/ND(a 1Δ) in solid Xe is of a different nature, the data being consistent with a temperature dependent external heavy atom effect. A lower limit of 1.9 s has been deduced for the radiative lifetime of the (a 1Δ) state in vacuo, in reasonable agreement with a recent ab initio calculation.