Polarization-modulation approach

A Digilab FTS-20E spectrometer with Nova 4 computer was used to record the spectra of the adsorbed species using polarization modulation approach (30). Detailed discussion of the instrumentation is given elsewhere (30). 

Frey, Bi., Com, RM., Weibel, S.C., 2006. Polarization-Modulation Approaches to Reflection-Absorption Spectroscopy. Handbook of Vibrational Spectroscopy. John Wiley Sons, Ltd. 

On metals in particular, the dependence of the radiation absorption by surface species on the orientation of the electrical vector can be fiilly exploited by using one of the several polarization techniques developed over the past few decades [27, 28, 29 and 30], The idea behind all those approaches is to acquire the p-to-s polarized light intensity ratio during each single IR interferometer scan since the adsorbate only absorbs the p-polarized component, that spectral ratio provides absorbance infonnation for the surface species exclusively. Polarization-modulation mediods provide the added advantage of being able to discriminate between the signals due to adsorbates and those from gas or liquid molecules. Thanks to this, RAIRS data on species chemisorbed on metals have been successfidly acquired in situ under catalytic conditions [31], and even in electrochemical cells [32]. 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

The combination of surface enhanced Raman scattering (SERS) and infrared reflection absorption spectroscopy (IRRAS) provides an effective in-situ approach for studying the electrode-electrolyte interface. The extreme sensitivity to surface species of SERS is well known. By using polarization modulation of the infrared beam for IRRAS, the complete band shape is obtained without modulating the electrode potential. 

For optimum performance, CD measurements require a polarization modulated source. In principle, any of the polarization-selective optical devices discussed earlier could be mechanically moved to create the required modulation. However, this approach is problematic in that it is difficult to implement physically, the mechanical movement may introduce noise into the measurement situation, and there are limitations to the rate at which the polarization can be modulated. A preferable approach is to use an electronic device to effect the required phase retardation. Although a number of devices have been used for this purpose (e.g. magneto-optical, Kerr effect, etc.), modern CD instruments rely upon either the Pockels effect, or photoelastic modulation for this function. 

The improvement in the instrumental SNR afforded by the use of polarization modulation has permitted CD detection to be applied to stopped-flow studies of biological reactions. The important information which can be obtained from such an approach has served as an impetus for the development of new instrumental approaches for the measurement of CD. These new approaches have allowed CD measurements to be extended to the time domain below that available with stopped-flow techniques. Presently, nanosecond, and even picosecond CD techniques have been developed, and it seems clear that extension to the sub-picosecond regime will follow. 

The technique using p-s modulation has received different names depending on the kind of IR instrument used. Thus for grating instruments it was called PMIRRAS (polarization modulation infrared reflection-absorption spectroscopy) [6]. For FT spectrometers the name FTIRRAS [8] was suggested. However this name was later used also in connection with Fourier transform spectra applying the potential difference approach. 

Equation 4 is applicable to the Type I experiment where there is circular polarization modulation at u)C. The relation between Aq and the sample s absorptivity and thermal properties is seen in Figure 3. A linear dependence on 3 and thus on g is found for < as (uB > us). At > as (ue < us) saturation occurs and the PACD magnitude decreases as the absorptivity increases. Thus photoacoustic saturation will lead to anomalously low g values, since the photoacoustic magnitude approaches a constant value for 3 > as (14). The PACD phase angle (Figure 4) is a function of as and 3 and varies from -90° (3 as) to 0° (3 as). 

One of the most commonly applied IR techniques developed to overcome these problems is the external reflectance technique. In this method, the shong solvent absorption is minimized by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The sensitivity problem, that is, the enhancement of the signal/noise ratio in the case of external reflectance techniques is solved by various approaches. These are, for instance, electrochemically modulated infrared spectroscopy (EMIRS), in situ FTIR (which use potential modulation), and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS, FTIR) [86,117-123]. 

A second lock-in detection method that was employed is polarization modulation, which involves modulating the polarization state of the incident infrared beam, and is again an extension of an approach developed for the study of the gas-solid interface [80]. Polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) relies upon the principles underlying the surface selection rule... 

PM-IRRAS exploits the different attenuation of s- and p-polarized light by adsorbed species at a reflective (electrode) surface to annul the unchanging contributions to the infrared signal at the detector from the solvent, window, and so on, and produces an absolute rather than difference spectrum at a particular potential. In this approach, a photo-elastic modulator is employed to modulate the polarization state of the incident infrared ray between s- and p-states. On the basis of Greenler s theory [81, 82], this polarization modulation gives rise to an AC signal at the detector, which is proportional (/p —7s)-the difference in intensity of the two polarizations. Since, in principle, /p is absorbed... 

The interference of water absorption is eliminated when the physicochemical and dynamic approaches are used for measuring the spectra, since a water response to a perturbation differs from that of a protein (vide supra). As an alternative, the spectrum can be rendered practically free from background absorption with a polarization modulation technique. To illustrate. Fig. 7.54 shows the spectrum from PM-IRRAS of an acetylcholinesterase (AChE) (enzyme) monolayer at the air-water interface at different surface pressures [838]. The spectra were... 

Concerning the limitation of this approach, one must consider that best results are obtained from infrared transmission spectra. This technique is well adapted for the study of solution (polymer-solvent acid—base interactions) or thin films (polymer—polymer add-base interactions). In the case of polymer—metal add—base interaction, one can prefer infrared reflexion-absorption spectroscopy (IRAS) or polarization-modulated infrared reflexion-absorption spectroscopy (PM-IRAS). Nevertheless this approach is a continuum-based (i.e., macroscopic) approach and it is important to rmderstand that adhesion forces are not specified explicitly even if acid-base interactions contributed to AJf . 

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