MOLECULAR THEORY OF 2ND-ORDER SUM-FREQUENCY GENERATION

Recently the second-order sum-frequency generation (SFG) has become a powerful surface - spectroscopic technique. In this paper we present a molecular theory for SFG. To systematically treat this process, we ha ve classified SFG into the resonance-resonance case, resonance-off-res onance case, off-resonance-resonance case and off-resonance-off-resona nce case. Among these cases we may have IR-UV SFG and UV-UV SFG. Some of these SFG's are reported in this paper. In each SFG case, we have s hown how to calculate the strength (or intensity) and band-shape funct ion.