Fig. 4
Sum frequency generation (SFG) spectroscopy. a SFG spectrometry. In the illustrated experiment, water-immersed polymer brush is examined, a dense collection of polymer chains on the surface of a quart prism cell. When two photons of frequencies (ω 1 and ω 2) were simultaneously provided for the sample, the SGF signal (ω SFG = ω 1 + ω 2) can emerge from water molecules whose inversion symmetry is broken. It seems probable that the nanointerface contains a number of these molecules. Note that the signal would not be generated from the water molecules that show random orientations. b SFG spectra of the OH stretching region of the MPC polymer brush using isotopic dilution water (HOD, H2O/D2O = 1/4) [23]