Transient absorption spectra of fac-Re(CO)3(CH3CN)3 + •N2 at several selected waiting times Tw are presented in Figs. 3a. For all spectra presented, the pump pulse was set to have double the amplitude compared to the individual probe pulses. Power-dependence measurements confirmed that transient signals were linear in pump power (Fig. ). Relatively strong bleaching signals were observed at the fundamental frequencies of the two normal modes. The total change in photodissociation yield compared to the linear response was around -15%, as measured from the raw time-domain data. Each frequency-resolved spectrum presented in Fig. was normalized to its respective frequency-resolved linear spectrum, which was simultaneously collected, to account for ion signal fluctuations between scans. Details about data processing procedures are provided in supplementary materials. The large nonlinear signals are not surprising given the very strong oscillator strengths of carbonyl stretches in metal complexes. As expected, ESA signals are not observed at the overtone frequencies.