POLARISATION MODULATION

The surface actlve/surface inactive difference between p-polarlsed/ s-polarised radiation has enabled an alternative modulation technique, polarisation modulation, to be developed (15,16). In electrochemical applications, it allows surface specificity to be achieved whilst working at fixed potential and without electrochemical modulation of the interface. It can be implemented either on EMIRS or on SNIFTIRS spectrometers and can be very valuable in dealing with electrochemically irreversible systems however, the achievable sensitivity falls well short of that obtained with electrochemical modulation. It should also be noted that its "surface specificity" is not truly surface but extends out into the electrolyte with decreasing specificity to about half a wavelength. 

Polarisation modulation infrared rejiection-absorption spectroscopy (PM-IRRAS or JRRAS). Potential modulation IR studies rely on switching the potential at a reflective electrode between rest and active states, generating difference spectra. However, the EMIRS technique has several drawbacks the relatively fast potential modulation requires that only fast and reversible electrochemical process are investigated the absorption due to irreversibly chemisorbed species would be gradually eliminated by the rapid perturbation. Secondly, there is some concern that rapid modulation between two potentials may, to some extent, in itself induce reactions to occur. 

Fig. 18.5. Comparison of the polarisation modulated (PM-IRRAS) spectra of linear COad, derived from adsorption of CO and of methanol from solution onto a Pt surface from, respectively, CO(sat)/0.5 M H2S04 and 1 M MeOH/O.5 M H2S04 at 0.4 V for 15 min. The polarisation modulation was conducted at 80 Hz by using a rotating polariser, and spectra are referred to 0.8 V. The total adsorbate coverage of 0.86 is common to the two spectra.

While the orientation of these deuterated chains shows a strong molecular weight dependence especially at short times, all chains exhibit a relatively constant residual orientation at long times. Even chains of molecular weight as low as 3000 present a non-zero local orientation. This was confirmed by polarisation modulation infrared dichroism - a technique sensitive to very small anisotropies [34]. 

PMFTIRRAS Polarisation modulated Fourier transform infrared reflection absorption spectroscopy... 

MOLECULAR ORIENTATION AND RELAXATION STUDIES OF POLYMERS USING POLARISATION MODULATION INFRARED SPECTROSCOPY... 

No.12, June 1994, p.2538-41 MOLECULAR ORIENTATION STUDIES IN POLYMER FILMS BY POLARISATION MODULATION FTIR SPECTROSCOPY... 

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