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<p>The temperature dependence of the vibrational sum-frequency generation (vSFG) spectra of the the air/water interface is investigated using many-body molecular dynamics
(MB-MD) simulations performed with the MB-pol potential energy function. The total vSFG spectra calculated for different polarization combinations are then analyzed
in terms of molecular auto-correlation and cross-correlation contributions. To provide
molecular-level insights into interfacial hydrogen-bonding topologies, which give rise to
specific spectroscopic features, the vSFG spectra are further investigated by separating
contributions associated with water molecules donating 0, 1, or 2 hydrogen bonds to
neighboring water molecules. This analysis suggests that the low frequency shoulder
of the free OH peak which appears at ∼3600 cm−1 is primarily due to intermolecular
couplings between both singly and doubly hydrogen-bonded molecules.
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Absolute frequency measurement of a SF_6 two-photon line by use of a femtosecond optical comb and sum-frequency generation
