The structure of an ${\mathrm{H}}_2\mathrm{O}$ monolayer bound to atomically smooth hydroxylated amorphous silica is probed under ambient conditions by near-infrared evanescent-wave cavity ring-down absorption spectroscopy. Employing a miniature monolithic optical resonator, we find sharp ($\approx 10{\mathrm{cm}}^{-1}$) and polarized ($>10:1$) vibration-combination bands for surface OH and adsorbed ${\mathrm{H}}_2\mathrm{O}$, which reveal ordered species in distinct local environments. Indicating first-monolayer uniqueness, the absorption bands for adsorbed ${\mathrm{H}}_2\mathrm{O}$ show intensity saturation and line narrowing with completion of one monolayer. Formation of the ordered ${\mathrm{H}}_2\mathrm{O}$ monolayer likely arises from H bonding to a quasicrystalline surface OH network.

• Received 4 May 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.166104