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 χ⁽³⁾ effect, which mainly includes induced molecular orientation by electric fields at charged interfaces. We theoretically evaluate the χ⁽³⁾ effect on the SFG spectroscopy of liquid water using molecular dynamics (MD) simulations. The MD simulations enable us to definitely calculate the χ⁽³⁾ susceptibility as a bulk property, and thereby separating it from the usual χ⁽²⁾ effect shown in the SFG spectra. The calculated results of χ⁽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 χ⁽³⁾ effect from bulk liquid.