Vibrationally-Resonant Sum Frequency Spectroscopy (VR-SFS):
A nonlinear vibrational spectroscopy in which two colors of light, one in the visible and one in the infrared, are mixed at an interface to create the sum frequency. The generated sum frequency light carries structural information about the molecules and chemical functionalities present at the interface.
As a second order nonlinear optical spectroscopy, VR-SFG has many unique features:
only technique to provide absolute orientation of molecules at surfaces
sensitive to interface layer only
order, protein secondary structure, and individual amino acid orientation
monitor conformational changes of polypeptides and proteins
These features make VR-SFG an exquisitely sensitive in situ probe of molecular structure.
Femtosecond lasers generate broadband light to probe 400-600 cm-1 regions of the infrared spectrum with each laser shot. Spectral resolution is controlled by the narrow bandwidth visible light. Mixing the infrared and visible light at an interface generates broadband light at the sum frequency. Multichannel detection allows collection of an entire spectrum in 1 minute. Lateral resolution is 100 μm.
VR-SFG probes the 2nd order nonlinear susceptibility, χ(2) , which has resonant components from the molecular vibrations and a non-resonant component from the interface itself. The interference between these components allows unique determination of molecular orientation.
Also note that for a vibrational mode to be SFG active, it must have both IR (μ) and Raman (α) optical activity.