Essentially, the nominal 3=1 anion and neutral levels are mixed with nearby vibrational states, so that the individual vibrational eigenstates are linear combinations of multiple harmonic zero-order vibrational states. A near-quantitative agreement with experiment is obtained using a newly developed implementation of vibrational perturbation theory (VPT) that identifies relevant anharmonic couplings among nearly degenerate vibrational states and provides the contributions of the individual harmonic states to the vibrational eigenstate. The underlying concept is illustrated in Figure . In the absence of anharmonic coupling, photodetachment from the anion 3=1 state Under these circumstances, the relative intensity of a transition between anion and neutral eigenstates