Optical Second Harmonic Generation as a Probe of Surface Chemistry

Optical second harmonic generation (SHG) is the nonlinear conversion of two photons of frequency w to a single photon of frequency 2w which, in the electric dipole approximation, requires a noncentrosymmetric medium. The ability of noncentrosymmetric crystals to produce SHG has led to their implementation as frequency doublers in a wide variety of laser systems. SHG can also be obtained from the break in symmetry that occurs at the interface between two centrosymmetric media. Since only the first few atomic or molecular monolayers on either side of the interface participate in this symmetry breaking, the SHG process can be used as a highly surface-selective optical probe of interfacial phenomena.' Shortly after the initial SHG measurements of Franken et al. in 1961,2 the sensitivity of SHG to the interface between two centrosymmetric media was experimentally demonstrated by Brown, Parks, and S l e e ~ e r . ~ In a series of papers, Bloembergen and coworkers determined the theoretical equations that govern SHG from surfaces in a reflection geometry,4g6 from a thin slab of nonlinearly active material: and finally from the interface of two centrosymmetric media.6 In the last case, it was assumed that the discontinuity in the electric fields at the interface led to a quadrupolar term in the nonlinear polarization that resulted in SHG from the interface. However, in 1969 Brown and Matsuoka demonstrated that the SHG from a silver surface was highly sensitive to the presence of adsorbed layers.' In this work, Brown and Matsuoka proposed that the observed sensitivity was due to an additional surface dipole term in the nonlinear polarization. Rudnik and Stern subsequently presented a new theoretical description of SHG from interfaces between centrosymmetric media in which the surface sensitivity of SHG was primarily attributed to the symmetry-breaking nature of the surface rather than the quadrupolar effects of the previous theories.8 From these initial works, SHG has evolved into a powerful surface technique that has been widely applied. The