By employing the nonlinear optical, interface selective experiment of sum frequency spectroscopy together with independent ab initio and density functional theory calculations, we determine the functional species of a corundum (001) surface: doubly coordinated OH groups which differ in their bond tilt angles. The interaction of the functional species with the adjacent water molecules is also observed. In a large pH range around the point of zero charge, the interaction is not controlled electrostatically but by hydrogen bonding. The functional species' tilt angles are crucial parameters, determining whether the species act as hydrogen bond donors or acceptors.

We introduce a novel nonlinear optical microscope which provides interface specific, chemically and structurally resolved infrared information with the improved spatial resolution limit of a visible light microscope. High impact on the investigation of a large variety of interfaces is expected. Here, the microscope is applied in a dewetting study.

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