Effects of third-order susceptibility in sum frequency generation spectra: a molecular dynamics study in liquid water

When sum frequency generation (SFG) spectroscopy is applied to charged solid/liquid interfaces, the observed SFG signals include both the second-order and third-order polarizations. The latter is called the chi((3)) effect, which mainly includes induced molecular orientation by electric fields at charged interfaces. We theoretically evaluate the chi((3)) effect on the SFG spectroscopy of liquid water using molecular dynamics (MD) simulations. The MD simulations enable us to definitely calculate the chi((3)) susceptibility as a bulk property, and thereby separating it from the usual chi((2)) effect shown in the SFG spectra. The calculated results of chi((3)) for liquid water are fairly consistent with the experimental estimates. The present finding is utilized to analyze the spectral change of SFG at the air/water interface under electric fields and at the charged silica/water interface. The present analysis of the spectral changes allows for distinguishing the intrinsic change of the interface structure and the chi((3)) effect from bulk liquid.