Equilibrium and Dynamic Processes at Interfaces by Sec()nd Harmonic and Sum Frequency Generation

Interfaces are ubiquitous. They occur everywhere. They are the boundaries that separate distinct physical and chemical regions of matter. The bulk materials on adjacent sides of the interface can be chemically identical, as with a liquid water/vapor water interface, or they can be chemically different, as at ~t liquid/liquid interface such as hexadecane/water. All transfers of chemical species between the bulk regions must cross the interface. But, aside from mediating all exchanges, the interface has unique chemical and physical properties, which differ from either bulk medium. Because of these properties, the interface has fundamental scientific and technological importance (1). The origin of the distinctive interface properties is the asymmetry in forces that molecules and atomic species experience there, together with the almost, two-dimensional geometry of the interface. These features are manifested in such things as the chemical composition at an interface. For example, even when the water concentration greatly exceeds that of alcohol in a bulk alcohol-water solution, we would find that the composition is almost exclusively alcohol at the vapor/solution interface. As with alcoholwater, the chemical compositions of the interface and the surrounding