The presence of vibronic coupling here is affirmed by the differing behavior of the FC-forbidden and FC-allowed features as the photon energy is changed, as shown in Figure . Here, cryo-SEVI spectra are plotted at three photon energies, showing that as the photon energy is lowered and approaches the detachment threshold, the intensity of features A1 and A2 drops precipitously relative to that of B1 and B3. This effect is ascribed to a reduced photodetachment cross section for feature A1 and A3 at low eKE. Such a difference can be related to the relative scaling of the detachment cross sections at low eKE's, given by the Wigner threshold law: 67