coupling to an excited electronic state with the appropriate symmetry. Briefly, two vibronic states, a and b, can mix through HT-coupling provided their electronic and vibrational symmetries elec Γ and vib Γ , respectively, satisfy 63 , each an admixture of the two zero-order vibronic levels. Thus, if detachment to state b is FC-allowed, then detachment to state a0 becomes allowed, but will display the electronic character of state b.