Heterogeneity of Free O--H Groups at the Air/Water Interface

Free O--H groups at the air/water interface produce a narrow peak in vibrational sum-frequency generation (vSFG) spectra, yet their orientational anisotropy is often overlooked. Using polarization-resolved vSFG experiments and path-integral quantum dynamics with the MB-pol potential, we show that the free O--H frequency depends on orientation: O--H bonds tilted toward the interfacial plane are red-shifted and reorient faster than more upright O--H. This shift originates from orientation-dependent local electric fields generated by neighboring water molecules, as demonstrated by a clear anti-correlation between the projected field along the O--H bond and the fundamental frequency. Combining polarization-resolved vSFG with quantum data-driven many-body simulations enables direct spectral interpretation in terms of orientation-dependent local electric fields, yielding a molecular-level picture of interfacial free O--H heterogeneity.