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The bulk and surface of a medium generally have different structural symmetries
so their optical responses obey different selection rules. Hence, optical probes
can be surface specific. Sum Frequency Generation Spectroscopy (SFG)
is an excellent technique for characterizing molecules at a surface or
buried interface. The great advantage of SFG over other vibrational
spectrscopic methods (i.e., IR) is its inherent nature in eliminating
interference from the bulk. It is highly sensitive
and capable of achieving high spatial and spectral resolution. This probe is capable of
determining molecular orientation and structural information at the interface of interest, and
can be applied to numerous areas in surface and materials chemistry. Our
group has successfully studied the structure and dynamics
of molecules at interfaces of atmospheric relevance,
such as sulfuric acid, sulfate salts, ammonia and HCl (publications).
SFG is a second-order, nonlinear optical process in which two pulsed laser beams, a fixed visible beam, wVIS, and a tunable infrared beam, wIR, are combined. A third frequency, wSF, is emitted at the sum of these input frequencies when they are temporally and spatially overlapped on a surface.
A vibrational spectrum is generated
as the tunable IR frequency comes into resonance with vibrational modes of
molecules at the interface. Surface specificity arises because SFG is allowed only where inversion symmetry is broken, thus forbidden in centrosymmetric media such as bulk liquids or gases. Therefore, SFG selectively probes species at the interface. The macroscopic and microscopic susceptibility tensors, c(2)and b(2) describe spectral features and peak intensities in SFG spectra. Their relationship depends on the orientation and number density of probed molecules with respect to the surface.
SFG Technique
Surface Studies of Liquid/Air Interfaces
Heterogeneous Stratospheric Chemistry on Liquid Surfaces
© 2001 Shultz Research Group
Last Updated: 3/4/04
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