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We propose a novel interpretation of the water liquid-vapor interface vibrational sum-frequency (VSF) spectrum in terms of hydrogen-bonding classes. Unlike an absorption spectrum, the VSF signal can be considered as a sum of signed contributions from different hydrogen-bonded species in the sample. We show that the recently observed positive feature at low frequency, in the imaginary part of the signal, is a result of cancellation between the positive contributions from four-hydrogen-bonded molecules and negative contributions from those molecules with one or two broken hydrogen bonds. Spectral densities for each of these subgroups span the entire relevant spectral range. Three-body interactions within our newly developed E3B water simulation model prove to be critical in describing the proper balance between different hydrogen-bonded species, as (two-body) SPC/E, TIP4P, and TIP4P/2005 models fail to reproduce the positive feature. The results clarify the molecular origin of the VSF signal, and highlight the importance of many-body interactions for water in heterogeneous situations.
Pieniazek et al. (Mon,) studied this question.