Solvent effects Raman optical activity

By analogy ivith SHG described above, if two light beams of fixed visible frequency cOvis and tunable IR frequency cOir are focused onto a surface then due to nonlinear optical effects reflected light of sum-frequency cOsf = cOvis-I-(Uir may be detected, hence the name sum-frequency generation. The selection rules demand that only bands that are both IR and Raman active are observed. Unfortunately, sum frequency generation (SFG) suffers from the disadvantage of strong IR absorption by solvent, so irradiation from the back of the electrode is sometimes used. 

---