Spin-forbidden radiative decay of the a 4Piu state of O⁺₂

Abstract

The spin-forbidden radiative decay of a 4Piu O⁺₂ has been measured in a radio frequency octopole ion trap. Photodissociation is used to probe the a 2Piu population as a function of trapping time. We have found that the a 4Piu state exhibits a multiple exponential decay, ranging from a few milliseconds to hundreds of milliseconds. The state dependence of the decay is seen in the photodissociation spectrum (b 4Sigma^-_g<--a 4Piu), which changes dramatically from 0.1 to 100 ms. The major changes in the spectrum are simulated by assuming that the F2 and F3 spin components of the a 4Piu state decay faster than the F1 and F4 components. We can account for this dependence on spin sublevel by assuming that the primary mechanism for radiative decay arises from spin-orbit coupling of the a 4Piu and A 2Piu states. Our results suggest that the a 4Piu radiative lifetime of 0.22 s measured by O'Keefe and McDonald reflects the decay of only the longest living a 4Piu sublevels.