The third trace in Figure ) is the linear response predicted by model fitting to the 2D IR waiting time series (scaled and offset to best match the spectrum measured with the probe beam). The strong match between the predicted response and probe spectrum is independent validation of model fitting. For comparison, Figure shows linear absorption fits to the FTIR spectrum for the CLS method. FTIR lineshapes can be unreliable near the baseline due to the difficulties with imperfect background subtraction, especially for dilute solutions of weak chromophores, and therefore, we show results for fitting to the upper 90%, 80% and 70% of the linear absorbance spectrum. We float the homogeneous lifetime, Kubo amplitude, linear scaling and offset as fitting variables while holding the Kubo correlation time constant at 3.3ps. We see that 80% provides a reasonable balance between quality of fit and distortions as a result of the baseline, which is in line with the recommendation by Kwak and coworkers. It is notable that the linear absorption predicted by model fitting in Figure is still a higher quality fit than the 90% case for the CLS analysis.