  Vibrational coupling is relevant not only for dissipation of excess energy
after chemical reactions but also for elucidating molecular structure and
dynamics. It is particularly important for OH stretch vibrational spectra of
water, for which it is known that in bulk both intra- and intermolecular
coupling alter the intensity and line shape of the spectra. In contrast with
bulk, the unified picture of the inter/intra-molecular coupling of OH groups at
the water-air interface has been lacking. Here, combining sum-frequency
generation experiments and simulation for isotopically diluted water and
alcohols, we unveil effects of inter- and intramolecular coupling on the
vibrational spectra of interfacial water. Our results show that both inter- and
intramolecular coupling contribute to the OH stretch vibrational response of
the neat H2O surface, with intramolecular coupling generating a double-peak
feature, while the intermolecular coupling induces a significant red shift in
the OH stretch response.
