T or -1 doesn't contribute much to T hom -1 , so we think this is a reasonable approximation for convenience, though model fitting to polarization dependent spectra to fit T or has been done. For comparison, we also analyze the simulated data using the center-line-slope (CLS) method. One feature of 2D Kubo lineshapes is that they are asymmetric in frequency and the asymmetry is, itself, frequency dependent (see Figure ). This effect leads to inaccurate centerline measurements when fitting the slices with a symmetric function. Therefore, we fit an asymmetric Lorentzian (Lorentzian + linear term + offset) to slices along the probe axis and then measure the peak of the asymmetric fitting function by numerically finding the extremum of the interpolated function to obtain the center-line points. This method provides a more accurate measure of the true centerline and perfectly agrees with Falvo's analytical expression for calculating the CLS decay by double integration of the response function when tested on our simulated spectra. Fitting a biexponential model to the CLS decay yields the Kubo time constants. For this analysis, we include all waiting time points, including T W = 0. We measure homogeneous dephasing and Kubo amplitudes by fitting to the upper 80% of the linear absorption spectrum holding the Kubo decay times constant. Many baseline distortions are known to occur in FTIR measurements including interference fringes, atmospheric absorption, and scattering from scratched windows, particles and aggregates. Any subtle variation between background and sample measurements, such as path length, index of refraction, concentration and location of particles or aggregates, window scratches, temperature, pressure, atmosphere, and other conditions, can lead to residual distortions in background-subtracted FTIR spectra. These distortions, in addition to various forms of noise, are nontrivial to model so we do not include them in our simulation. In fact, to consider the best-case scenario for the CLS, we fit to a noiseless simulation of the linear absorption spectrum. Nevertheless, we note that in real samples, these contributions would all lead to even greater uncertainty for the lineshape parameters from the CLS method. Panel (B) shows the results for an individual trial (selected at random) with 95% confidence intervals derived from the covariance of the fit in Eq. 7. Corresponding plots of (B) for every single trial (Video S2) confirm that, for the model fitting approach, the true values fall within the confidence intervals for ~95% of trials. In contrast, the CLS method grossly underestimates the confidence intervals for the Kubo amplitudes and homogeneous dephasing. In calculating these confidence intervals for the CLS method, it was assumed that the Kubo time constants were known with absolute certainty, which is currently the standard treatment. In reality, the Kubo time constants obtained from CLS fits are uncertain, with reported errors ranging from 5-50%. This additional uncertainty must be accounted for when fitting the linear absorption spectrum if we hope to achieve accurate uncertainties for homogeneous dephasing and Kubo amplitudes. Accounting for this effect involves propagating the uncertainty of the Kubo time constants through the standard variance-covariance matrix of the linear absorption fit to obtain the modified variance-covariance matrix (Eq. S14), which we derive in supporting information section E. To the best of our knowledge, CLS error bars have never been reported using Eq. S14.