Polarization-modulation techniques

RAIRS is a non-destructive infrared technique with special versatility - it does not require the vacuum conditions essential for electron spectroscopic methods and is, therefore, in principle, applicable to the study of growth processes [4.270]. By use of a polarization modulation technique surfaces in a gas phase can be investigated. Higher surface sensitivity is achieved by modulation of the polarization between s and p. This method can also be used to discriminate between anisotropic near-sur-face absorption and isotropic absorption in the gas phase [4.271]. 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

Figure lb. Schematic diagram of polarized light used with the polarization modulation technique. (Reproduced with permission from Ref. 9. Copyright 1984 Elsevier.)... 

Small degrees of circular polarization (V/I) in starlight, usually less than 1%, have been observed in recent years the polarization modulation technique for observing such polarization was discussed in Section 13.7. There are at least two mechanisms for circularly polarizing starlight. 

Jasperson, S. N., and S. E. Schnatterly, 1969. An improved method for high reflectivity ellipsome-try based on a new polarization modulation technique, Rev. Sci. Instrum., 40, 761-767. 

Over conventional reflectance photometry, IRES retains advantages 1) and 2), with 3) being partially lost (the substrate contribution to A only is measurable) however, sensitivities are a factor of 10 or more lower. Polarization modulation techniques that determine only Ae, and not the corresponding dispersion property (6,J), retain advantage 1) over wavelength modulation photometry,but not advantage 2),and retain advantage 3) only in part, as above. 

In the following overview of further typical examples, no attempt is made to distinguish between results obtained via potential and polarization modulation techniques. An early overview of investigated systems is available [199]. 

Various modem accessories (ATR crystals, acoustic detectors, infrared microscopes, polarization modulation technique) as well as hyphenation techniques have substantially expanded the field of application of infrared spectroscopy. Applications of IR spectroscopy to surface investigations (characterization of the surface, physi-sorption and chemisorption studies, catalytic properties) are reviewed in [7-9]. Applications of hyphenated techniques, in particular combinations with chromatography, are given in [8]. 

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... 

The power of this technique can be illustrated in the measurement of segmental orientation when polymers are deformed macroscopically. Despite the tremendous improvement of signal-to-noise ratio available with the Fourier transform instrument, it has not been possible to measure extremely small changes in segment orientation as a function of strain. The usual polarization measurements involve two separate measurements, one parallel and one perpendicular, followed by calculation of their difference to obtain the dichroic ratio, and then the calculation of the orientation function (eq. 13). For nearly isotropic materials this type of measurement is of marginal value. However, with the polarization modulation technique it is possible to obtain extremely accurate polarization changes when samples are mechanically deformed. Extremely accurate polarization measurements can be obtained in this way, even for amorphous segments in poly(butylene terephthalate-co-tetramethylene oxide) (236). 

Ellipsometry Based on a New Polarization Modulation Technique, Rev, Sci Instrum., 40(1969) 761-767. 

Polarization Modulation in FTIR Spectroscopy (PM-FTIRS). Recently, Kunimatsu, Golden, and coworkers have coupled the polarization modulation technique with Fourier transform infrared spectroscopy and applied the new method to electrochemical studies (Figure 20). In their work, a double modulation technique is employed, using a photomodulator working at high frequency (78 kHz) and a polarizer, both inserted in the optical path. 

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