Mapping molecular orientation and conformation at interfaces by surface nonlinear optics

Second-order nonlinear optics can be used to quantitatively determine the orientation of chemical bonds or submoieties of a fairly complicated molecule at an interface, and therefore completely map out its orientation and conformation. As a specific example, we have studied pentyl-cyanoterphenyl molecules at the air-water interface. We have measured the orientation of all three parts of the molecule (cyano head group, terphenyl ring, and pentyl chain) by optical second-harmonic generation and infrared-visible sum-frequency generation. A quantitatively consistent picture of the molecular configuration has been obtained. The technique can be applied to situations where other methods would fail (e.g., the surface of neat liquids or buried interfaces). [S0163-1829(99)04119-3].