PM IRRAS

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

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. 

Hence, another approach is required that will enable the study of a wider range of electrochemical processes. Two other techniques are potentially capable of fulfilling this role PM-IRRAS and in situ FTIR. 

A great deal of success was attendant on the early application of PM-IRRAS to the gas/solid interface. Golden et ai (1981) reported the development of instrumentation, using conventional dispersive optics, able to record detailed infrared reflection-absorption spectra from molecules adsorbed on single-crystal Pt without any interference from the gas-phase species. 

A schematic representation of the instrumentation used in the in s/fu FT1R technique is shown in Figure 2.49. As can be seen from the figure, the instrumentation is much simpler than that required to perform EMIRS or PM-IRRAS measurements. 

Figure 2. PM-IRRAS (left) and SERS (right) spectra for Ag electrode in 0.01M cyanide solution for various potentials, -0.4V to -1.4V (Ag/AgCl).

Figure 9. PM-IRRAS spectra for Ag electrode in 0.03 M azide in 0.1 M Na01O4. These are obtained by taking the difference of the spectra taken at the specified potential and at -0.95 V (Ag/AgCl). (Reprinted with permission from ref. 50. Copyright 1988 American Institute of Physics.)...

Vibrational spectra were recorded using the polarization-modulated infrared reflection absorbance technique (PM-IRRAS). The spectrometer, the electrochemical cell, and the sample preparation and cleaning procedures are all described elsewhere (1 7) All of the measurements were performed using 0.5 M SO solutions, either with or without an added nitrile compound or SnCl,. The solutions were saturated with CO by bubbling the gas through their storage reservoirs before admitting them into the sample cell. 

In Figure 1 we show the PM-IRRAS spectra for a Pt electrode exposed to saturated C0/H S0 solutions which contain various concentrations of different organic nitriles. For comparison, we have also included a spectrum recorded in saturated CO/H SO with no added nitrile. The adsorption step was accomplished by pulling the electrode back into the bulk solution and cycling the potential from 0.55 V(SHE) up to 1.15 V, down to 0.0 V, and back to 0.55 V. The spectra were recorded after re-positioning the electrode against the cell window while the potential was held at 0.55 V. 

Figure 1. PM-IRRAS spectra of CO adsorbed on Pt in 0.5 M H S0, at 0.55 V(SHE) in the presence of different added nitrile compounds.

PM-IRRAS Polarization-modulated infrared reflertion absorption specTroscopy... 

A variant of IRRAS is polarization modulation IR reflection absorption spectroscopy (PM-IRRAS). In this method, the polarization of the IR beam incident on the sample is modulated between parallel and perpendicular polarization. When the sample is metallic, only the parallel-polarized light yields signals from adsorbed molecules, because the electric field amplitude of perpendicular-polarized light vanishes at the metal surface. This statement is the basis for the metal surface selection rule 100,109). When the medium above the sample (gas or liquid phase) is isotropic, both polarizations are equivalent. The PM-IRRAS method thus enables the measurement of signals from adsorbates on a metal surface in the presence of an absorbing gas or liquid phase. 

The method has been applied, for example, in electrochemical investigations (110) and also for surface catalytic reactions in the presence of a gas phase 111). When PM-IRRAS is used with a thin-layer cell, as depicted in Fig. 37, the contribution from dissolved molecules in the liquid phase can be minimized. Still, the layer thickness has to be small to prevent complete absorption of the IR radiation by the solvent. The combination of polarization modulation and ATR for metal films was demonstrated recently and applied in an investigation of self-assembled octadecylmercaptan monolayers on thin gold films 112). This combination could emerge as a valuable technique for the investigation of model catalysts. 

This review includes the structural information that has been carried out to date from films of single chain amphiphiles, enzymes, and proteins using both IRRAS and PM-IRRAS methods. 

POLARIZATION MODULATION REFLECTION ABSORPTION SPECTROSCOPY (PM-IRRAS) OF LANGMUIR FILMS... 

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. 

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