Perspectives on surface sum-frequency spectroscopy.

This paper explores possible improvements to surface-specific sum-frequency spectroscopy toward a more versatile and powerful surface probe. Several suggestions are made to broaden the applicability of the technique, including the introduction of dual-comb sum-frequency spectroscopy, which is expected to greatly enhance the signal strength. The paper also highlights existing confusion and misinterpretation in spectral analysis used to interrogate interfaces. To address this issue, a framework is suggested that defines material response coefficients extractable from measured spectra in terms of averages of microscopic quantities that characterize an interface. Such response coefficients can be theoretically calculated for comparison with the extracted ones. Comments on works in three areas that have garnered considerable attention in recent years are given. It is seen that these results are either incorrectly presented, misinterpreted, or overstated.