Infrared-Visible Sum Frequency Spectroscopy
SFG experiment a pulsed tunable infrared IR (wVIR) laser beam is mixed with a visible VIS (wVIS) beam to produce an output at the sum frequency (wSFG = wVIR + wVIS).
SFG is second-order nonlinear process, which is allowed only in media without inversion symmetry. At surfaces or interfaces inversion symmetry is necessarily broken, that makes SFG highly surface specific.
As the IR wavelength is scanned, active vibrational modes of molecules at the interface give a resonant contribution to SFG signal. The resonant enhancement provides spectral information on surface characteristic vibrational transitions. Different combinations of input and output beam polarizations allow the determination of surface symmetry or molecular orientation.
SFG detects vibrational modes, which are rather localized to specific groups of atoms within the molecules. Information about relative orientation of different groups within the same molecule maybe obtained, and, hence, the molecular structure can be deducted.
A view on our SFG spectrometer