We assigned the vibronic peaks by comparing to the Franck-Condon simulation. To this end, the spectrum is simulated once again, this time, however, setting the linear diabatic constant to zero (λ= 0). This yields the spectra of the two electronic states at an equivalent level of theory but without vibronic coupling, i.e., the combined Franck-Condon spectra of the two states. Figure shows this spectrum. The non-coupled spectrum is easy to assign using the labels that mark the symmetry of the electronic state, A 1 or B 2 , and the vibrational state (ν 1 ν 2 ν 3 ), where ν i is the number of quanta in mode i (i = 1 for the symmetric stretch, i = 2 for the symmetric bend and i = 3 for the asymmetric stretch). The assigned spectrum shows progressions in the symmetric bend. There is one such progression in each electronic state. Additionally for each state, there is also another progression in the symmetric bend with one excitation in the symmetric stretch.