Fundamentals of SNIFTIRS

In SNIFTIRS, the electrode potential is modulated between the base value (Ei) and the sample value E2), and the spectra of the reflected infrared radiation are measured. The reflection absorption spectrum (AR/R) is obtained by plotting 

Because of the subtraction, SNIFTIRS spectra are devoid of the common background signal from the aqueous electrolyte. However, they represent the difference between absorbances of organic molecules at potentials Ei and E2 (or precisely the difference multiplied by 2.3). In order to facilitate interpretation of such spectra, it is convenient to choose a value of the base potential Fi at which the molecules are totally desorbed into the bulk of the thin-layer cavity. The sample potential E2 corresponds then to the adsorbed state of the film. Consequently, SNIFTIRS spectra plot a difference between the absorbances of molecules desorbed into the thin-layer cavity at potential Ei and those adsorbed at the electrode surface at potential 2- Thus, positive bands (or positive lobes in bipolar bands) are due to absorbance by desorbed molecules and negative bands (or negative lobes in bipolar bands) are the result of absorbance by adsorbed molecules. 

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