Infrared polarization modulation technique

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. 

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]. 

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. 

I. Noda, A.E. Dowrey and C. Marcott, Characterization of polymers using polarization-modulation infrared techniques Dynamic infrared linear dichroism (DIRLD) spectroscopy. 

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. 

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. 

For the measurement of differential absorption spectra in the visible and ultraviolet spectral region, a light modulation technique was introduced over two decades ago [7]. The principle of this measurement also governs the measurement of infrared VCD. This technique involves the high frequency modulation of the exciting beam between left and right circular polarization states via a photoelastic modulator, and measuring the differential absorption with a lock-in amplifier tuned to the modulation frequency. 

Boujday S, Methivier C, Beccard B, Pradier C-M (2009) Innovative surface characterization techniques applied to immunosensor elaboration and test comparing the efficiency of Fourier transform-surface plasmon resonance, quartz crystal microbalance with dissipation measurements, and polarization modulation-reflection absorption infrared spectroscopy. Anal Biochem 387 194-201... 

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. 

During this work, we analyzed the organization, the morphology, and the preferential adsorption of these copolymers and how these properties would be influenced by the interfacial interactions that could be established between the polymer and the chemical functionalities grafted onto the adsorption substrate. Our results were mainly based on two surface techniques polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and atomic force microscopy (AFM). 

The chemical identity of molecular adsorbates on electrode surfaces can be derived from their vibrational behavior. In situ infrared (IR) and Raman spectroscopy are possible. Because of the strong IR absorption of most electrolyte solvents, modulated techniques are necessary. Potential and polarization modulation have been employed [53] and the methods are named correspondingly. Modulation is not nec-... 

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]. 

The second means of increasing the S/N ratio, which has also been employed with FTIR spectrometers, involves modulating the polarization state of the light, a technique termed polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). This technique relies on a subsidiary selection rule, valid... 

Noda, L Dowrey, A. E. Marcott, C., Characterization of Polymers Using Polarization-Modulation Infrared Techniques Dynamic Infrared Linear Dichroism (DIRLD) Spectroscopy. In Fourier Transform Infrared Characterization of Polymers, Ishida, H., Ed. Plenum Press New York, 1987 pp 33-57. 

Although optical vibrational techniques are less sensitive than electron-based spectro-metric methods, these techniques are employed extensively for thin-film characterization because of the specific and characteristic vibrational spectrum shown by various functional groups and molecules present in the film. The most commonly used vibrational spectroscopic techniques are infrared (IR) and Raman spectroscopy. Because of the interference caused by absorption of IR by the underlying substrate, IR reflection-adsorption spectroscopy (IRRAS) and its polarization modulation (PM) analog, PM-IRRAS, which uses the polarization selectivity of surface adsorption, are typically employed to characterize thin films (Gregoriou and Rodman, 2006). 

Despite the fact that measurement of spectra with both types of light and subsequent ratioing should be simple, so far no report of measurements done with polarization modulation has appeared. This is a common technique in infrared spectroscopy at surfaces (see subsequent text). 

That is, the properties of liquid/solid or gas/Uquid interfaces cannot be investigated properly with XPS. On the other hand, in case of using photon-in and photon-out techniques, e.g., sum frequency generation (SFG) spectroscopy [15] or polarization modulation infrared reflection-absorption spectroscopy (PM-IRAS) [16, 17], it is possible to look into various surfaces and interfaces under elevated pressure condition or even reaction conditions, including liquid/solid interface. However, the interpretation of its result is often not straightforward, and it often requires the advanced understanding to extract information, e.g., the difficulty in quantitative analysis. 

Polarized FTIR spectroscopy has been used extensively to study polymer orientation (i.e., the dichroic ratio and dichroic difference are normally obtained from spectra recorded sequentially with the infrared radiation polarized parallel and perpendicular to a reference direction). To improve the sensitivity of this technique, and to follow accurately the dynamics of orientation, FTIR spectroscopy has been coupled with a polarization modulation (PM) technique whereby the dichroic difference spectrum is recorded directly, thus minimizing instrumental and sample fluctuations (this is discussed later in the chapter). 

---