Fourier Transform Raman Spectrometer

FIGURE 4.19 Schematic representation of a conventional Raman spectrometer. 

The Physical Chemistry of Materials Energy and Environmental Applications 

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The Fourier transform Raman spectrometer is constructed around an interferometer (see Figure 4.20) [57], Normally, a continuous wave Nd YAG laser (1064nm) is used for the sample excitation. In relation to the sample arrangement inside the spectrometer, there are two fundamental geometries in which a sample is tested in Raman spectroscopy, that is, the 90° geometry, where the laser beam... 

The Raman spectrum of gases can now also be recorded with Fourier-Transform Raman spectrometers with near infrared excitation (Dyer and Hendra, 1992). Fig. 4.3-19 shows a survey spectrum of air obtained in 4 hours of sampling time (Bruker, 1993). The region of the rotational spectrum is presented on an expanded scale in Fig. 4.3-20, it can be compared with Fig. 4.3-18. The intensities of the lines below about 80 cm are weakened by the Rayleigh line suppression filter and the resolution is limited to 1 cm", mainly by the laser used for excitation. 

Due to the rather stringent requirements placed on the monochromator, a double or triple monochromator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm, and the linewidths are only tens of cm j, it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monochromator to collect the spectrum. In fact, this is the approach taken with Fourier transform Raman spectrometers. 

Dispersive Raman spectrometers are used with excitation in the visible range (typically He—Ne or Ar+ lasers are used), Fourier transform Raman spectrometers are used with excitation in the near infrared range (Nd YAG laser). For both ranges, microscopic techniques working with a laser beam diameter of micrometer size, are commercially available. 

The high temperature (330C) cure reaction of4-phenoxy-4 -phenylethynylbenzophenone was monitored using a modulated fibre-optic Fourier transform-Raman spectrometer. Spectral evidence for two different reaction pathways (cyclisation and chain extension) is provided. The formation of a conjugated double bond system of varying chain lengths is indicated. 19 refs. 

FT-Raman instrument, 488,491, 492. See also Fourier transform, Raman spectrometers... 

The basic methods of the identification and study of matrix-isolated intermediates are infrared (IR), ultraviolet-visible (UV-vis), Raman and electron spin resonance (esr) spectroscopy. The most widely used is IR spectroscopy, which has some significant advantages. One of them is its high information content, and the other lies in the absence of overlapping bands in matrix IR spectra because the peaks are very narrow (about 1 cm ), due to the low temperature and the absence of rotation and interaction between molecules in the matrix. This fact allows the identification of practically all the compounds present, even in multicomponent reaetion mixtures, and the determination of vibrational frequencies of molecules with high accuracy (up to 0.01 cm when Fourier transform infrared spectrometers are used). 

Conventional Raman spectroscopy cannot be applied directly to aqueous extracts of sediments and soils, although it is occasionally used to provide information on organic solvent extracts of such samples. Fourier transform Raman spectroscopy, on the other hand, can be directly applied to water samples. The technique complements infrared spectroscopy in that some functional groups, eg unsaturation, give a much stronger response in the infrared. Several manufacturers (Perkin-Elmer, Digilab, Broker) now supply Fourier transform infrared spectrometers. 

Vibrational spectroscopy is an important tool for the characterization of various chemical species. Valuable information regarding molecular structures as well as intra- and intermolecular forces can be extracted from vibrational spectral data. Recent advances, such as the introduction of laser sources to Raman spectroscopy, the commercial availability of Fourier transform infrared spectrometers, and the continuing development and application of the matrix-isolation technique to a variety of chemical systems, have greatly enhanced the utility of vibrational spectroscopy to chemists. 

Detailed experimental procedures for obtaining infrared spectra on humic and fulvic acids have been reported previously 9,22,25-26) and will be briefly described here. Infrared spectra were taken on the size-fractionated samples by using a Fourier transform infrared spectrometer (Mattson, Polaris) with a cooled Hg/Cd/Te detector. Dried humic and fulvic materials were studied by diffuse reflectance infrared spectroscopy (Spectra Tech DRIFT accessory) and reported in K-M units, as well as by transmission absorbance in a KBr pellet. Infrared absorption spectra were obtained directly on the aqueous size-fractioned concentrates with CIR (Spectra Tech CIRCLE accessory). Raman spectra were taken by using an argon ion laser (Spectra-Physics Model 2025-05), a triple-grating monochromator (Spex Triplemate Model 1877), and a photodiode array detector system (Princeton Applied Research Model 1420). All Raman and infrared spectra were taken at 2 cm resolution. 

The Fourier transform Raman (FT-Raniiin) instrument uses a Michelson interferometer, similar to that used in FTIR spectrometers, and a eontinuoits-wave (CW) Nd-YA(i laser as shown in Figure 18-12. fhe use of a 1064-nm (1.064-pm) source virtually eliminates fluorescence and photodecomposition of samples. Hence, dyes and other fluorescing compounds can be investigated with I T-RarniUi instruments. T he Ff-Raman instrument also provides superior Irequency precision relative to conventional instruments, which enable spectral subtractions and high resolution measurements. 

Fourier Transform Raman (FT-Raman) Spectrometer In the mid-1980s, another type of multichannel Raman system was introduced. By exciting with a near-1R laser, good-quality Raman spectra can he recorded with a near-IR... 

Raman and infrared spectra have also been compared for a series of 1,4-benzodiazepines, including diazepam (Vallium) and of closely related compounds [16,17]. The complementary nature of these two vibrational spectroscopic techniques was highlighted and the data provided spectral features that allowed identification of the drugs. The value of Fourier transform Raman spectroscopy using a near-infrared excitation source was also demonstrated for these heterocyclic molecules which have a tendency to fluoresce with visible radiation from conventional dispersive Raman spectrometers. 

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