The theoretical investigation of sum and difference frequency generation in thin surface layers with rotational symmetry leads to formulas which connect the generated light intensities to the surface second order nonlinear susceptibility tensor. A maximum of seven tensor components can be determined in the case of lowest symmetry. Measurements in transmission should be especially useful since they allow easy variation of both polarization and angle of incidence. On the other hand, large signal enhancements are expected for total internal reflection geometries. A consistent set of Z t2) tensor components for a thin layer of rhodamine-6G adsorbed on fused silica is found based on data from reflection and transmission measurements.

Using a powerful, excimer-laser-pumped broadband coherent anti-Stokes Raman scattering (CARS) system, we have observed considerable changes in the populations of room-temperature nitrogen under atmospheric pressure. Pump-induced populations in the first, second, and third vibronic bands of N2 at room temperature resulted in an apparent vibrational temperature of 2550 K. The implications for broadband N2 CARS thermometry are discussed.

Second harmonic emission from a dye covered quartz surface has been observed in total internal reflection. Coverages with the dye nile-blue A yielded an increase in second harmonic output by several orders of magnitude as compared to coverages with rhodamine 6G. In the latter case only the harmonic light is at resonance with the S o-s 2 transition of the adsorbed dye molecules, whereas for nile-blue A both, the fundamental and the harmonic frequency, are at resonance with the respective singlet transitions.

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