The OH and CH regions of glycerol/water mixtures in the concentration range 0.0-1.0 mole fraction have been investigated with vibrational sum frequency generation. Glycerol is found to partition to the surface of these solutions in all concentrations. Neat glycerol surfaces contain no free OH groups projecting into the vapor. The surface orientation of glycerol is constant through most of the concentration range.

Structural data about water at the air/water interface of sulfuric acid solutions have been obtained with sum frequency generation spectroscopy. Sulfuric acid significantly affects the orientation of water at the interface. With as little as 0.01x (mole fraction) sulfuric acid, water is more highly oriented compared with the pure water surface. Surface water with one hydrogen free of hydrogen bonding, a “free OH”, decreases in concentration as the sulfuric acid concentration is increased. Finally, there are no sulfuric acid free OH groups projecting out of the surface for solutions from 0.01x to 0.9x sulfuric acid. Observations of the structure of water are consistent with the fact that sulfuric acid/water solutions change from ionic in nature at low concentrations (<0.01x) to ion pair complexes/hydrates at high concentrations (>0.4x).

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Consider sum frequency generation (SFG) ω3 = 2 ω1 + ω2 in a phase matched metal-noble gas mixture, using two lasers ω1 and ω2, where 2 ω1 is tuned to a two photon transition Ωg2 and ω2 is shifted across Rydberg transitions Ω2n (g groundstate, 2 excited state, n Rydberg state of the metal atom). Then the intensity of the SFG spectrum I3 (ω3)~ |χ(3) (ω3)|2 displays resonances and related antiresonances, separated by (1) which are caused respectively by constructive and destructive interference of resonant and nonresonant parts of the 3. order nonlinear susceptibility χ(3) (ω3 ≃ Ωng) = χR(ω3)+ χNR (3) The products μ2nμng:= xn of the dipole transition matrix elements may then be obtained from a simple measurement of the frequency spacings Δωn at a large number of transitions Ωgn and solving iteratively the coupled equation system (1),(3); neither absolute radiation intensity nor vapour density deter mination is needed.