The normalized spectra of PSAs are shown in the upper panel of Figure . The PSA analysis shows that the peak widths and peak centers from the 2D IR spectra collected in the fast-scan BoxCARS geometry closely match those from the spectra collected in the pump-probe geometry. The PSAs extracted from the fast-scan BoxCARS spectra have negligible background noise so the peaks have similar baselines and lineshapes at all waiting times. PSAs at additional waiting times are included in Figure . However, the background noise in the pump-probe geometry becomes prominent with increasing waiting times, which distorts the background as well as the lineshapes. The improvement in the background is also investigated by the projections of the 2D IR spectra onto the detection axis (3) as shown in the lower panel of Figure . The positive and negative peaks in the probe-axis projection plots correspond to the positive and negative peaks in the 2D IR spectra. The projections are normalized with respect to the maximum of the positive peaks. Note that the ratio of the negative and positive peak intensities is ~-1 for the fast-scan BoxCARS geometry but is ~-0.8 for the pump-probe geometry. Due to small inaccuracies in beam alignment and the routing of the LO around the sample, the relative ratio of the negative and positive peaks is different between the two geometries. The projections from the pump-probe spectra have greater background noise as indicated by the baseline fluctuations. Similar to the PSA analysis above, the fast-scan BoxCARS geometry gives relatively baseline-free projection lines and the preserved lineshapes for the main peaks even at long waiting times. Plots of probe-axis projections at additional waiting times are included in Figure . The improved background noise level from the fast-scan BoxCARS spectra can also be seen in the diagonal slice comparison across select waiting times in Figure . Signal-to-noise ratios (SNR) were calculated for each waiting time and compared between fast-scan BoxCARS and pump-probe geometries (Figure ) in order to quantify the improvement in signal. The SNR is defined as the ratio between the maximum amplitude and the standard deviation of the baseline noise in the PSAs (Figure ). The SNRs decrease with increasing waiting time for both geometries, but the SNR from the fast-scan BoxCARS was greater at all measured waiting times. The larger standard deviation in SNR from the fast-scan BoxCARS 2D IR PSAs at early waiting times can be attributed to the vertical elongation of the peak shapes and phase instability, as previously discussed. The ratio between the SNR of the fast-scan BoxCARS and that of pump-probe was calculated for each waiting time (Figure ), which shows a 3× improvement in SNR for the fast-scan BoxCARS spectra versus the pump-probe geometry. This factor of 3 indicates that 9× more measurements would be necessary for the pump-probe geometry to achieve similar SNR of fast-scan BoxCARS within this current configuration. It is worth noting that data acquisition in both geometries was done with the same setup. The fast-scan BoxCARS geometry can be converted to the pump-probe geometry by using the tracer as the probe beam and blocking one of the pump beams, the E3 beam, and the LO beam. Thus, this pump-probe setup after conversion does not have the full power for the Disclosures. The authors declare no conflicts of interest.