Polarization-modulated IR reflection

A long disputed issue of the nature of strongly bound species in this reaction has been recently revived with the vibrational spectroscopy studies of Bewick et al. (30) using EMIRS technique and of Kunimatsu and Kita (31) using polarization modulation IR-reflection-absorption technique. These data indicated the only CO is a strongly bound intermediate. Heitbaum et al. (32) on the other hand advocate COH, and most recently HCO (33), as the poisoning species on the basis of differential electrochemical mass spectroscopy (DEMS). 

In situ studies of catalytic reactions have also been a prime focus of our group. The high-pressure spectroscopic technique used in our research is polarization modulation IR reflection absorption spectroscopy (PM-IRAS). Like SFG, PM-IRAS is a highly surface-sensitive technique that yields vibrational information about adsorbed surface species. Unlike SFG, however, PM-IRAS... 

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. 

First, surface-sensitive techniques that can operate under technologically relevant conditions, i.e., at least in the 1 —lOOOmbar pressure range, are required. In this respect, photon-based techniques such as sum frequency generation (SFG) and polarization-modulation IR reflection absorption spectroscopy (PM-IRAS) provide surface vibrational spectra of adsorbates from UHV up to atmospheric pressure. Although electron spectroscopies are typically limited to pressures <10 mbar, recent developments in XPS allow the determination of... 

B. Polarization- Modulation IR Reflection Absorption Spectroscopy B.l. Basics... 

Fig. 10. Schematic illustration of polarization-modulated IR reflection absorption spectroscopy (PM-IRAS).

For further improvement, the reaction mechanism should be clarified. Although many reports have been published so far on the reaction kinetics, information obtained from the experiments is limited, and because of the variety of reaction models the reaction mechanism still remains unclear. The development of in situ observation technique will be necessary. Recently, some efforts were reported on in situ techniques. Lu et al. [6] applied infrared emission spectroscopy to observe the adsorbed species on a (Sm,Sr)Co03 cathode under operation. They suggested 02 is the most probable adsorbate (Fig. 7.2). Murai et al. [7] employed polarization-modulated IR reflection absorption spectroscopy and found response in a similar frequency region. Quantum mechanical calculations are also made by several researchers [8]. 

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. 

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. 

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]. 

Infrared Spectroscopy. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) data were collected using a Nicolet MAGNA-IR 860 Fourier transform spectrometer equipped with a liquid nitrogen-cooled mercury-cadmium-telluride (MCT) detector and a Hinds Instruments PEM-90 photoelastic modulator. The p-polarized light was incident at 80° from the surface normal. The spectra were collected for 1000 scans at a spectral resolution of 4 cm . ... 

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. 

Ishida H, Ishino Y, Buijs H, Tripp C and Dignam M J 1987 Polarization-modulation FT-IR reflection spectroscopy using a polarizing Michelson interferometer App/, Spectrosc. 41 1288-94... 

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). 

In this chapter, a sensitive method for measurement by continuous-scan Fourier-transform infrared (FT-IR) spectrometry called polarization-modulation spectrometry is introduced this is a useful method for measuring, with high signal-to-noise ratio, not only the reflection-absorption spectra of thin films adsorbed onto metal substrates but also other spectra such as vibrational circular dichroism (VCD) spectra. Polarization-modulation spectrometry is a type of double-modulation FT-IR spectrometry [1], In this chapter, descriptions of double-modulation spectrometry are given first, then polarization-modulation spectrometry is discussed, and then its application to the measurement of reflection-absorption spectra of thin films on metal substrates is discussed. 

Figure 11,9 Block diagram of a polarization-modulation FT-IR spectrometer set-up for measuring reflection-absorption spectra from thin films on metal substrates.

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