Polarity/polarization second harmonic generation spectroscopy

H. Wang, E. Borguet, and K. B. Eisenthal, The polarity of liquid interfaces by second harmonic generation spectroscopy, J. Phys. Chem. 101 (1997) 713. 

H. Watanabe, S. Yamaguchi, S. Sen, A. Morita, and T. Tahara,/. Chem. Phys., 132, 144701 (2010). Half-Hydration At the Air/Water Interface Revealed by Heterodyne-Detected Electronic Sum Frequency Generation Spectroscopy, Polarization Second Harmonic Generation, and Molecular Dynamics Simulation. 

The symmetry of the LB films was determined by polarized ultraviolet-visible (UV-Vis) absorption spectroscopy, optical rotation, and second-harmonic generation. All studies showed that the constructed LB films are anisotropic in the plane of the film and that the symmetry of the film is C2 with the twofold rotation axis perpendicular to the film plane. For example, when the SH intensity is plotted as a function of the azimuthal rotation angle (rotation around an axis perpendicular to the plane of the film), the twofold symmetry becomes evident (Figure 9.23). Isotropic films generate an SH signal independent of the azimuthal rotation angle. On the other hand, the LB... 

We have obtained SERS and second harmonic generation (SHG) from the doublehole structures with different hole distances. The dependence of the SHG efficiency on the spacing between the holes in the basis is shown in Fig. 11 for two polarizations of the fundamental beam. It can be seen that, as expected from the calculations shown in Fig. lOd, the SHG signal maximizes when the two holes overlap [63,64]. This optimum distance has also been observed in SERS experiments using the double-hole basis [65]. The double-hole structure offers two degrees of freedom, shape and periodicity, for electric field optimization and enhanced spectroscopy. 

A prerequisite for the development indicated above to occur, is a parallel development in instrumentation to facilitate both physical and chemical characterization. TEM and SPM based methods will continue to play a central role in this development, since they possess the required nanometer (and subnanometer) spatial resolution. Optical spectroscopy using reflection adsorption infrared spectroscopy (RAIRS), polarization modulation infrared adsorption reflection spectroscopy (PM-IRRAS), second harmonic generation (SFIG), sum frequency generation (SFG), various in situ X-ray absorption (XAFS) and X-ray diffraction spectroscopies (XRD), and maybe also surface enhanced Raman scattering (SERS), etc., will play an important role when characterizing adsorbates on catalyst surfaces under reaction conditions. Few other methods fulfill the requirements of being able to operate over a wide pressure gap (to several atmospheres) and to be nondestructive. 

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