Resolution of the Ã/B̃ photoionization branching ratio paradox for the 12CO+2 B̃(000) state

Abstract

The interaction between the ¹²CO⁺₂ B̃(000) level and vibronic levels of the à state has been investigated. The detailed nature of this mixing proves to be the key to the quantitative understanding of the Ã/B̃ electronic state branching ratio discrepancy in which photoelectron spectroscopy gives about one‐third the value of the Ã/B̃ ratio as that derived from fluorescence measurements. Upon excitation of rotational lines of the ¹²CO⁺₂ B̃(000)–X̃(000) band near 2900 Å, part of the resulting dispersed fluorescence is found to be shifted to the red (λ>3000 Å). Detailed high resolution spectroscopic studies using jet‐cooled ions and population labeling optical–optical double resonance show that this red shift is caused by perturbations of the B̃(000) state by two states belonging to the òΠ_u manifold. When the red‐shifted (to λ>3300 Å) fluorescence quantum yield for the perturbed B̃(000) state is summed over all populated rotational levels of the excited state, this gives 0.37±0.05, in excellent agreement with that previously obtained from photoelectron–photon coincidence measurements following photoionization of CO₂ by the He 584 Å resonance line. After corrections for emission occurring in the region 3000 <λ< 3300 Å, the total quantum yield for emission at λ>3000 Å from the B̃(000) level is found to be 0.42±0.07. These results show that the principal mechanism responsible for the Ã/B̃ branching ratio paradox is interaction of specific rotational states of the B̃(000) level and perturbing levels of the à state of ¹²CO⁺₂.