Polarization modulation IRRAS

Fig.4.53. Experimental and simulated PM (polarization modulated) IRRAS spectra of single monolayers of (A) PEG and (B) K(LK)7 at the air-water interface. The surface pressure was 20 mN m [4.281],...

Kunimatsu K, Aramata A, Nakajima N, Kita H. 1986. Infrared spectra of carbon monoxide adsorbed on a smooth gold electrode Part II. EMIRS and polarization-modulated IRRAS study of the adsorbed CO layer in acidic and alkaline solutions. J Electroanal Chem 207 293-307. 

Li J.-T, Maurice V., Swiatowska-Mrowiecka J., Seyeux A., Zanna S., Klein L., Sun S.-G., Marcus P. XPS, time-of-flight-SIMS and polarization modulation IRRAS study of Cr203 thin film materials as anode for lithium ion battery, Electrochim. Acta 2009, 54,3700-3707. 

In the usual manifestation of this experiment, the velocity of the interferometer mirror must be low enough that the conventional interferogram can be filtered out before the signal is input to the lock-in amplifier. In addition, the amplitude of the PEM must be set at a certain value, so that for only one wavelength in the spectrum is the polarization of the output radiation at the extreme of the cycle rotated by exactly 90°. A method of performing polarization-modulation IRRAS... 

Xiao, Y., Bourque, H Pezolet, M., and Ritcey, A.M. (1998) Polarization modulation IRRAS investigation of a cellulose ether at the air/water interface. Thin Solid Films, 327-329,299. 

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. 

Kunimatsu K, Golden WG, Seki H, Philpott MR. 1985a. Carbon monoxide adsorption on a platinum electrode studied by polarization modulated FT-IRRAS. 1. Co Adsorbed in the double-layer potential region and its oxidation in acids. Langmuir 1 245 -250. 

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. 

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. 

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. 

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. 

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

A prerequisite for the development indicated above to occur, is a parallel development in instrumentation to facilitate both physical and chemical characterization. TEM and SPM based methods will continue to play a central role in this development, since they possess the required nanometer (and subnanometer) spatial resolution. Optical spectroscopy using reflection adsorption infrared spectroscopy (RAIRS), polarization modulation infrared adsorption reflection spectroscopy (PM-IRRAS), second harmonic generation (SFIG), sum frequency generation (SFG), various in situ X-ray absorption (XAFS) and X-ray diffraction spectroscopies (XRD), and maybe also surface enhanced Raman scattering (SERS), etc., will play an important role when characterizing adsorbates on catalyst surfaces under reaction conditions. Few other methods fulfill the requirements of being able to operate over a wide pressure gap (to several atmospheres) and to be nondestructive. 

PM-IRRAS Polarization-Modulated Infra-Red Reflection Adsorption Spectroscopy FTIRS Fourier Transform Infra-Red Spectroscopy... 

SNIFTRS Subtractively Interfacial Fourier Transform Infra-Red Spectroscopy PM-FT-IRRAS Polarization Modulated Fourier Transform Infra-Red Reflection Absorption Spectroscopy... 

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). 

Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS)... 

Polarization-modulation infrared reflection-absorption spectroscopy (PM-IR-RAS) spectra were recorded with a Bruker ITS 66/S Fourier transform infrared spectrometer equipped with a PMA 37 polarization modulation module and a ititrogen-cooled MCT detector. The infrared beam was first p-polarized with a ZnSe wire grid polarizer (Specac) before passing through a photoelastic modulator (Hinds Instruments, PEM-90), which modulated at a frequency of 74 kHz. A lock-in ampHfier (Stanford model SR-830) was used to obtain the PM-IRRAS spectra. The half-wave retardation frequency was set at 4000 cm . The PM-IR-RAS spectra were recorded as S= R -Rs)/(R +Rg). A total of 250 scans at a resolution of 4 cm were collected for each measurement at an angle of incidence of 82.5° with respect to the normal to the sample surface. 

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... 

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... 

It is noteworthy here that some bands of anisotropic films on transparent substrates can vanish from the jp-polarized IRRAS [69,70] or polarization modulation (PM) IRRAS [71, 72] spectra. This phenomenon is a consequence of the SSR for dielectrics (see Section 3.11.4 for more detail). 

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