Studies of Bonding and Reaction on Metal Surfaces Using Second-Harmonic and Sum-Frequency Generation

Over the past 10 years there has been a resurgence in efforts to develop nonlinear optical techniques for studies of surfaces and interfaces. Chief among these techniques are second-harmonic generation (SHG) and ir-visible sum-frequency generation (SFG). These second-order nonlinear processes offer several advantages. They are inherently sensitive only to the interfacial region between centrosymmetric media. This is because, in the electric dipole approximation, these processes are forbidden in the bulk of such media but are allowed at the interface where the symmetry is broken. Thus, so-called buried interfaces, i.e. surfaces under liquids, high pressure gases, and even other solids, may be investigated. In addition, with sufficiently short-duration laser excitation, these techniques have the potential to provide sub-picosecond time resolution.