We note three interesting features of this lower frequency response at early waiting time. First, as shown in Videos S8 and S9, the phase of the signal oscillates as a function of waiting time with a ~1.3 ps period (~25 cm -1 ). Second, the signal initially appears stretched along the diagonal and dephases with a lifetime of 1-2 ps. Third, as shown in both videos, the intensity of the residual response is roughly 10% of the peak 2D IR signal in both 2020 and 2021 data, which implies a third-order response. We suspect this may be a resonantly enhanced wave packet of low-frequency Raman modes (e.g. the methyl torsion) anharmonically coupled to the C≡N stretch, similar to what has been seen in a variety of other oscillators. At longer waiting times, the residual appears as a vertical peak shift of the 0-1 transition coincident with the vibrational lifetime, which is characteristic of hot ground state absorption. Figure (A) shows the linear absorbance spectrum for 2021 data obtained using the probe beam and upconversion spectrometer (black, solid) and using an FTIR (blue, dashed). We subtracted DMSO backgrounds in both spectra, offset and scale the FTIR spectrum to best match the probe spectrum via linear least-squares, but the FTIR spectrum remains notably narrower than the probe spectrum. We attribute this to different instrument response functions between the two measurements. For example, the FTIR spectrum is influence by factors such as apodization, scan length and vignetting of light along moving optics, while the probe spectrum is influenced by the resolving power of the spectrometer and the bandwidth of the 800 nm pump utilized in upconverting the infrared light prior to the spectrometer.