The technique was developed in 1987 by Yuen-Ron Shen and his students as an extension of second harmonic generation spectroscopy and rapidly applied to deduce the composition, orientation distributions, and structural information of molecules at gas–solid, gas–liquid and liquid–solid interfaces.
[1][2] Soon after its invention, Philippe Guyot-Sionnest extended the technique to obtain the first measurements of electronic and vibrational dynamics at surfaces.
[3][4][5] SFG has advantages in its ability to be monolayer surface sensitive, ability to be performed in situ (for example aqueous surfaces and in gases), and its capability to provide ultrafast time resolution.
SFG gives information complementary to infrared and Raman spectroscopy.
[6] IR-visible sum frequency generation spectroscopy uses two laser beams (an infrared probe, and a visible pump) that spatially and temporally overlap at a surface of a material or the interface between two media.
[7] Broadly speaking, most sum frequency spectrometers can be considered as one of two types, scanning systems (those with narrow bandwidth probe beams) and broadband systems (those with broad bandwidth probe beams).
For the former type of spectrometer, the pump beam is a visible wavelength laser held at a constant frequency, and the other (the probe beam) is a tunable infrared laser — by tuning the IR laser, the system can scan across molecular resonances and obtain a vibrational spectrum of the interfacial region in a piecewise fashion.
These laser beams overlap at a surface, but may access a wider range of molecular resonances simultaneously than a scanning spectrometer, and hence spectra can be acquired significantly faster, allowing the ability to perform time-resolved measurements with interfacial sensitivity.
It is worth noting that all the even order susceptibilities become zero in centrosymmetric media.
As a second-order nonlinear process, SFG is dependent on the 2nd order susceptibility
Centrosymmetric media include the bulk of gases, liquids, and most solids under the assumption of the electric-dipole approximation, which neglects the signal generated by multipoles and magnetic moments.
[7] At an interface between two different materials or two centrosymmetric media, the inversion symmetry is broken and an SFG signal can be generated.
This suggests that the resulting spectra represent a thin layer of molecules.
The constant of proportionality varies across literature, many of them including the product of the square of the output frequency,
[6] The above equations can be combined to form which is used to model the SFG output over a range of wavenumbers.
When the SFG system scans over a vibrational mode of the surface molecule, the output intensity is resonantly enhanced.
[6][9] In a graphical analysis of the output intensity versus wavenumber, this is represented by Lorentzian peaks.
Depending on the system, inhomogeneous broadening and interference between peaks may occur.
[13] From the second order susceptibility, it is possible to ascertain information about the orientation of molecules at the surface.
describes how the molecules at the interface respond to the input beam.
As a rank 3 tensor, the individual elements provide information about the orientation.
rod symmetry, only seven of the twenty seven tensor elements are nonzero (with four being linearly independent), which are The tensor elements can be determined by using two different polarizers, one for the electric field vector perpendicular to the plane of incidence, labeled S, and one for the electric field vector parallel to the plane of incidence, labeled P. Four combinations are sufficient: PPP, SSP, SPS, PSS, with the letters listed in decreasing frequency, so the first is for the sum frequency, the second is for the visible beam, and the last is for the infrared beam.
By taking the tensor elements and applying the correct transformations, the orientation of the molecules on the surface can be found.
[6][9][13] Since SFG is a second-order nonlinear optical phenomenon, one of the main technical concerns in an experimental setup is being able to generate a signal strong enough to detect, with discernible peaks and narrow bandwidths.
Picosecond and femtosecond pulse width lasers are often used due to the high peak field intensities.
Common sources include Ti:Sapphire lasers, which can easily operate in the femtosecond regime, or Neodymium based lasers, for picosecond operation.
Whilst shorter pulses results in higher peak intensities, the spectral bandwidth of the laser pulse is also increased, which can place a limit on the spectral resolution of the output of an experimental setup.
This can be compensated for by narrowing the bandwidth of the pump pulse, resulting in a tradeoff for desired properties.
[13] Signal strength can be improved by using special geometries, such as a total internal reflection setup which uses a prism to change the angles so they are close to the critical angles, allowing the SFG signal to be generated at its critical angle, enhancing the signal.
[13] Common detector setups utilize a monochromator and a photomultiplier for filtering and detecting.