Spin-forbidden transitions in the presence of an intersystem crossing: application to the b 1Σ + state in OH +

Abstract

A procedure to calculate spin-forbidden radiative transitions in the presence of an intersystem crossing occurring in either the initial or final state has been developed and applied to determine the lifetime of the b 1Σ + state in OH +. Potential curves for many states and dipole moments and dipole transition moments between those as well as spin—orbit matrix elements have been evaluated as a function of OH + separations by employing ab initio MRD CI wavefunctions. It is found that the higher vibronic levels (υ > 5) of the b 1Σ + state are predissociated by the A 3Π state, whereby the lifetime is in the order of 2 × 10 −12 s, i.e. the transition probability for intersystem crossing is around 4 × 10 11 s −1. In the lower levels depopulation occurs via the spin-forbidden b 1Σ + 0−X 3Σ ± 1 mechanism whereby the transition probability is in the order of 10 4 s −1. The corresponding parallel transition is weaker by a factor of roughly 10 3. The probability for the dipole-allowed A 3Π− 3Σ − process is 3 × 10 5 s −1 or τ = 3.3 × 10 −6 s.