Theory of PM-IRRAS on Dielectrics

As discussed above, the IRRAS method still lacks sensitivity for the study of a monolayer spread on water it seems to necessitate running successive long-time accumulations of many interferograms with a covered and uncovered water surface. Moreover, the strong absorption of the water vapor hides the spectral region where the most interesting molecular information is. In order to 

On a metallic substrate, PM increases the surface absorption detectivity of IRRAS by several orders of magnitude and provides high-quality monolayer spectra that can be quantitatively analyzed in terms of orientation and conformation of the surface molecules in a few minutes [85-88]. Moreover, due to the differential nature of the detected signal, these spectra are independent of the isotropic IR absorptions of the sample environment and water vapor interference is diminished. For these particular reasons, it appeared interesting to adapt PM-IRRAS method to the study of a monolayer spread at the air-water interface. 

Basically, in general PM-IRRAS experiment on any sample with polarized reflectances Rp and Rs, the signal at the detector output can be electronically split into a first part carrying only the intensity modulation induced by the moving mirror of the Fourier transform infrared (FTIR) spectrometer  

That in addition contains the PM induced by the photoelastic modulator [85,86], 

After demodulation, the ratio of these two parts gives the so-called PM-IRRAS signal  

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