Fig. 7: Humidity-dependent SFG spectra of hydrous species on glass samples at different relative humidity values: 0% (a), 40% (b), 90% (c).

The spectra were measured under ssp polarization combination and normalized to IR and visible beam intensities. The solid lines are fitted curves obtained from non-linear fittings of SFG spectra. The fit results are shown in Supplementary Table 4 in the SI. The plot in d shows oscillator strength ratio of 3200 cm−1 band (A3200) and 3400 cm−1 band (A3400) at 40 and 90% RH.