Sum and Difference Frequency Generation

Because mostly vibrational modes at interfaces are investigated, the method has also been called vibrational sum frequency generation (VSFG). 

COad formed by chemisorption of methanol on a platinum electrode has been studied with SFG (see Fig. 5.134) [786]. 

The spectra show two resonances located around 2065 cm and 1965 cm . The first corresponds to linearly bound CO, the latter was assigned to CO in a bridge-bound position [786]. The conversion of CO in a bridging location into species atop has been identified with SFG [794]. The combination of the high light intensity (e.g. free-electron lasers) and the possibility to obtain absolute spectra without any modulation technique has allowed the detection of generally weak bands like those of over- and underpotential deposited hydrogen on a platinum electrode [795, 796]. Studies of coadsorption of cyanide anions and cetylpyridinium cations on Au(l 11) and Au(210) revealed marked differences [797]. CN is bound 

Cetylpyridinium is a popular electrolyte solution additive in electroplating baths. 

Approaches to interpretation of results based on quantum mechanical calculations have been described [806]. The interface between a platinum electrode surface and a room temperature ionic liquid (l-butyl-3-methyUmidazolium tetrafluorobo-rate) in the presence of adsorbed CO has been probed with SFG [807]. The position of the electrode potential of zero charge pzc could be estimated and reorientation of adsorbed ions as a function of appUed electrode potential could be deduced. 

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A. Tadjeddine, A. Le Rille, Sum and difference frequency generation at electrode surfaces in Interfacial Electrochemistry. Theory, Experiment and Applications (Ed. A. Wieckowski), Marcel Dekker, New York-Basel, 1999, p. 317. 

Xe(2)(-ffl3,ffll,ffl2) Three wave mixing, sum and difference frequency generation... 

Here Xo (2w), x (wiiw2) and Xo (O) are the non-resonant values of the hyperpolarisabilities. Thus second harmonic generation is resonantly enhanced at both the fundamental and the harmonic of the optical transition, sum and difference frequency generation at the fundamentals and the sum and difference frequencies, and the rarely observed optical rectification only at the fundamental frequency. The term 3 in the expansion gives rise to effects such as third harmonic generation, x(3) -3oj oj, oj,u>), electric field induced second harmonic generation, x(3) (- 2w 0,w, oj), the optical Kerr effect, x(3) (-oj oj, oj, -cj), etc. that will display resonances at oj, 2oj and 3u>. 

For sum and difference frequency generation, the u>i and u>2 beams are superimposed using a dichroic mirror. The phase matching condition is... 

Sum and difference frequency generation at electrode suifaces [130] The investigation of vibrational and electronic properties of the electrochemical interface by using nonlinear optical techniques of visible-infrared sum (SFG) and difference (DFG) frequency generation constitutes an interesting aspect of the study of adsorbed species. The vibrational behavior of H-Pt(hkl) system, both in the underpotential and overpotential regime, adsorption of CN on Au(hkl) electrode (using DFG), electrode surface electronic properties were studied and measurements of the PZC were carried out [131]. 

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