Spin-forbidden radiative decay of the a ⁴Π_u state of O₂⁺

The spin-forbidden radiative decay of a ⁴Π_u O ₂⁺ has been measured in a radio frequency octopole ion trap. Photodissociation is used to probe the a ²Π_u population as a function of trapping time. We have found that the a ⁴Π_u 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 ⁴Σ_g^-←a ⁴Π_u) , 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 ⁴Π_u 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 ⁴Π_u and A ²Π_u states. Our results suggest that the a ⁴Π_u radiative lifetime of 0.22 s measured by O'Keefe and McDonald reflects the decay of only the longest living a ⁴Π_u sublevels.