Fourier transform infrared modulation

Dowrey A E and Maroott C 1982 A double-modulation Fourier transform infrared approaoh to studying adsorbates on metal surfaoes Appl. Spectrosc. 36 414-16... 

Earner B J, Green M J, Saez E I and Corn R M 1991 Polarization modulation Fourier transform infrared... 

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. 

It is only since 1980 that in situ spectroscopic techniques have been developed to obtain identification of the adsorbed intermediates and hence of reliable reaction mechanisms. These new infrared spectroscopic in situ techniques, such as electrochemically modulated infrared reflectance spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier transform infrared reflectance spectroscopy, or a subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS), have provided definitive proof for the presence of strongly adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic poisons. " " Even though this chapter is not devoted to the description of in situ infrared techniques, it is useful to briefly note the advantages and limitations of such spectroscopic methods. 

It can be readily seen from the above discussion that a simplistic version of the polarization modulation method for using Fourier Transform Infrared method would be to say that if one subtracted the message obtained from the vertical light from that obtained from the parallel light, the result will be information from the surface only. The relevant equation is shown below. 

A Fourier transform infrared spectroscopy spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference signals measured at the detector are usually amplified and then digitized. A digital computer initially records and then processes the interferogram and also allows the spectral data that results to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

This restilt was confimed by other groupsi2.i3, using Fourier transform infrared spectroscopy. Kunimatsu and Kita made further progress using polarization modulation to enable quantitative measurements and showed... 

The most recent advance in VCD instrumentation has been its adaptation to Fourier transform infrared (FTIR) measurement (23-25). The details of this technique involve a new method of FTIR measurement termed double-modulation FTIR spectroscopy. Thus spectra of very high quality and resolution have been obtained using a standard VCD modulator and detector, a glower source, and a commercially available FTIR spectrometer system. In fact an entire FTIR-VCD spectrometer can be assembled from a few commercially available components. It is found that the major advantages of resolution, throughput, and... 

PAS spectra are similar to those obtained using ordinary Fourier transform infrared (FTIR) spectroscopy except truncation of strong absorption bands which occurs due to photoacoustic signal saturation. PAS allows the structure to be studied at different thicknesses because the slower the frequency of modulation, the deeper the penetration of IR radiation. 

Fourier-transform infrared (FTIR) spectrometers encode infrared wavenumbers by moving a mirror in a Michelson interferometer which results in a unique, path-dependent pattern of interference for each light wavelength in the IR beam. FTIRs have come to totally dominate the IR market and are the means by which most of the work described in this review was accomplished. Only for some special applications (modulation spectra and time-dependence studies) are dispersive-based (scanning monochromator or tuned laser) spectrometers still used. The advantages of the FTIR approach are that the entire spectral region of interest can... 

Although VCD and ROA are presently measured in entirely different ways, future configurations of the two kinds of instruments may draw them into closer correspondence. For instance, measurement of ROA using a Fourier transform infrared spectrometer, with polarization modulation developed within the interferometer, has been proposed as a possible way to measure ROA [43]. 

The next class of VCD instruments to be developed was centered around a Fourier transform infrared (FT-IR) spectrometer. The idea was to design the sample compartment to be the same as in a dispersive VCD instrument, including a photoelastic modulator. To measure VCD, the detector signal is first sent to a lock-in amplifier to demodulate the high-frequency polarization modulation. The output of the lock-in is a VCD interferogram which is Fourier transformed in much the same way as the ordinary transmission interferogram. 

Principal component analysis is most easily explained by showing its application on a familiar type of data. In this chapter we show the application of PCA to chromatographic-spectroscopic data. These data sets are the kind produced by so-called hyphenated methods such as gas chromatography (GC) or high-performance liquid chromatography (HPLC) coupled to a multivariate detector such as a mass spectrometer (MS), Fourier transform infrared spectrometer (FTIR), or UV/visible spectrometer. Examples of some common hyphenated methods include GC-MS, GC-FTIR, HPLC-UV/Vis, and HLPC-MS. In all these types of data sets, a response in one dimension (e.g., chromatographic separation) modulates the response of a detector (e.g., a spectrum) in a second dimension. 

Basically, in general PM-IRRAS experiment on any sample with polarized reflectances Rp and Rs, the signal at the detector output can be electronically split into a first part carrying only the intensity modulation induced by the moving mirror of the Fourier transform infrared (FTIR) spectrometer ... 

In spectral interferometry, the interference in the spectral domain is exploited. The spectral modulation period is essentially determined by a time delay. This is at the heart of Fourier transform infrared spectrometers (FTIRs). 

Volume 50 of Advances in Catalysis, published in 2006, was the hrst of a set of three focused on physical characterization of solid catalysts in the functioning state. This volume is the second in the set. The hrst four chapters are devoted to vibrational spectroscopies, including Fourier transform infrared (Lamberti et al.), ultraviolet Raman (Stair), inelastic neutron scattering (Albers and Parker), and infrared-visible sum frequency generation and polarization-modulation infrared rehection absorption (Rupprechter). Additional chapters deal with electron paramagnetic resonance (EPR) (Bruckner) and Mossbauer spectroscopies (Millet) and oscillating microbalance catalytic reactors (Chen et al.). 

PSII = Photosystem II WOC = Water-oxidizing complex OEC = Oxygen-evolving complex (B)RC = (Bacterial) Reaction Center Chi = Chlorophyll Bchl = Bacteriochloro-phyll XRD = X-ray diffraction EPR = Electron paramagnetic resonance EXAFS = Extended X-ray absorption fine stmctnre ENDOR = Electron-nuclear double resonance ESEEM = Electron spin echo envelope modulation (Tyr = Yz) = DlTyrl61 ATP = Adenosine Triphosphate KIE = Kinetic isotope effect UV = Ultraviolet (FT-)IR = (Fourier Transform) InfraRed. 

The shallow hydrogenlc donors, on the other hand, have small binding energies and also have small central-cell corrections. This makes the resolution of different donors resulting from different chemical impurities difficult to achieve. The early experiments from which different chemical donors were identified employed hlgh-resolutlon Fourier-transform infrared magnetospectroscopy (FTIR) which used the modulated photoconductivity detection technique to monitor the 1S-2P transition In a fixed magnetic field. 

Figure 2 gives a scheme illustrating some applications in geochemistry and technology where surface reactivity (kinetics of dissolution, catalytic activity, photochemical activity) depends on surface structure, expecially on surface coordination. It has been shown by various spectroscopic techniques [electron-spin resonance (ESR), electron double-resonance spectroscopy (ENDOR), electron-spin echo modulation (e.g., see Motschi, 1987), Fourier transform infrared spectroscopy (Zeltner et al., 1986), and in situ X-ray absorption studies of surface complexes (EXAFS) (Hayes et al., 1987 Brown, 1989)] that inner-sphere... 

Tenn. Electronic spectroscopy was performed on a Varian Cary 17 recording spectrophotometer and a Nicolet Fourier Transform Infrared instrument was used to record spectra between 4000 and 200 cm l of samples in Csl. Electrical measurements were performed at room temperature on compacted samples by a four probe technique. TGA was performed on a DuPont 990 Analyzer using a DuPont 951 TGA module. TGA for the GC/MS analysis was performed on a DuPont 950 instrument. Volatile products were collected in a Tenax containing tube which was attached to the GC inlet port, GC/MS was performed on a Hewlett Packard 5982A coupled to a 5934A data system. DSC was performed on a DuPont 1090 analyzer using a DuPont 910 DSC module. 

Pons and co-workers [79] reported the first potentially modulated in-situ FTIR studies of the near-electrode region and they then developed the technique [24, 55, 56, 69] and eventually coined the acronym SNIFTIRS (subtractively normalised interfacial Fourier transform infrared spectroscopy). Corrigan et al. [81-83] and Bockris and co-workers [80, 84-88] have also reported studies employing the technique, or variations on it. These techniques all employ some form of potential modulation regime as with EMIRS, intended to cancel out all those absorptions that do not change with potential (bulk solvent, window, etc.), the spectra are again presented as (AR/R)v vs. v. However, the stepwidths (i.e. the time spent at each potential) in a SNIFTIRS experiment are much longer than those in EMIRS several tens of seconds instead of a tenth of second [89, 90]. 

Barner B J, Green M J, Saez E I and Corn R M 1991 Polarization modulation Fourier transform infrared reflectance measurements of thin films and monolayers at metal surfaces utilizing real-time sampling electronics Anal. Chem. 63 55-60... 

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