Figure 3: Vibrational relaxation time constants of bulk and interfacial H2O.
Experimentally observed relaxation times τ1 of the OH stretch vibration of bulk (blue symbols) and interfacial (red symbols) H2O as a function of the OH stretch excitation frequency. The open-red symbol corresponds to the vibrational relaxation time of the free (non hydrogen-bonded) OH groups. Error bars for the infrared pump-probe decay times correspond to a 100% increase in the sum of the squared deviations of the fit of the kinetic model to the data shown in Supplementary Figs 2 and 3. Error bars for the infrared pump-SFG-probe decay times correspond to the standard error obtained using a Levenberg–Marquardt fit of a single exponential decay to the experimental data in Fig. 2b. The solid blue curve represents the τ1 time calculated using the model with the overtone of the bending vibration centred at 3,250 cm−1. The dashed (dotted) red line shows the τ1 time calculated using the same model with the bending overtone of interfacial water centred at 3,190 cm−1 and a twofold (threefold) reduced spectral diffusion rate (for details, see text and Methods section).
