IR spectra were computed from the ratio of untagged ions to the sum of tagged and untagged ions multiplied by laser power, as a function of wavelength. UV spectra displayed in Figure , were computed by dividing the integrated intensity of fragment ions by the sum of the integrated intensity of fragment and parent ions and then divided by the measured laser pulse energy. UV spectra reported in Figure are recorded by monitoring the photofragment only, as it is not possible for the reflectron to focus both fragment and parent ions simultaneously for all fragments due to their kinetic energy differences. This reduction in kinetic energy also results in reduced detection efficiency of the fragments. A method to correct for this effect is described in the Supplementary Information.