IR and laser Raman spectroscopy

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

Poole and Finney (1983b), using direct difference IR and laser Raman spectroscopy, studied the hydration of lysozyme. On the basis of these measurements, they suggested that there are conformational changes just below the hydration level for the onset of enzyme activity (i.e., 0.2-0.25 h). This conclusion conflicts with that of Careri et al. (1979b). Poole and Finney (1984) extended these measurements to lactalbumin. 

Hatsuo Ishida is presently Assistant Professor of Macromolecu-lar Science at Case Western Reserve University and is also Director of C. Richard Newpher Pol3mier Composite Processing Laboratory which has recently been established at this university. He received his Ph.D. in Macromolecular Science from the same university where he applied FT-IR and laser Raman spectroscopy to his research on composite materials. He has been active in the molecular studies of glass flber/matrlx Interface of composites and his latest Interest Involves the correlation of molecular structure and mechanical property of composites made by various processing conditions. 

Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

The vibrations of molecular bonds provide insight into bonding and stmcture. This information can be obtained by infrared spectroscopy (IRS), laser Raman spectroscopy, or electron energy loss spectroscopy (EELS). IRS and EELS have provided a wealth of data about the stmcture of catalysts and the bonding of adsorbates. IRS has also been used under reaction conditions to follow the dynamics of adsorbed reactants, intermediates, and products. Raman spectroscopy has provided exciting information about the precursors involved in the synthesis of catalysts and the stmcture of adsorbates present on catalyst and electrode surfaces. 

Laser Raman spectroscopy is well suited for the study of air-sensitive liquids because the sample may be contained in an all-glass cell.21 Such a cell is much easier to load on a vacuum line and to maintain leak-free than is an infrared cell. Also, such a tube is easier to heat or cool than the typical IR cell. 

Individual and non-destructive chemical analysis of microscopic remnants enclosed in mineral grains has been proven possible by certain spectroscopic techniques such as Raman-, IR-, UV/visible- and Laser mass spectroscopy (Pflug, 1982)16>. 

Pulsed laser-Raman spectroscopy is an attractive candidate for chemical diagnostics of reactions of explosives which take place on a sub-microsecond time scale. Inverse Raman (IRS) or stimulated Raman loss (.1, ) and Raman Induced Kerr Effect (2) Spectroscopies (RIKES) are particularly attractive for singlepulse work on such reactions in condensed phases for the following reasons (1) simplicity of operation, only beam overlap is required (2) no non-resonant interference with the spontaneous spectrum (3) for IRS and some variations of RIKES, the intensity is linear in concentration, pump power, and cross-secti on. 

The origin of Raman spectra is markedly different from that of IR spectra. In Raman spectroscopy, the sample is irradiated by intense laser beams in the UV-visible region (v0), and the scattered light is usually observed in the direction perpendicular to the incident beam (Fig. 1-7 see also Chapter 2,... 

In principle, laser Raman spectroscopy provides complementary data to IR spectroscopy, but this technique is difficult to apply successfully because of its lower sensitivity, the high fluorescence background of some supports, and the potential destruction of the sample by the incident laser radiation. Raman spectroscopy was used to provide structural evidence for metal-metal and metal-oxygen bonds on the surface-bound clusters such as [HjRejfCOlij]" on MgO (752) and [HOs3(CO)uOM=](M=Si, Al) on SiO and AljOj (20). 

In Raman spectroscopy, the light is nearly monochromatic and is usually in the visible range. The light source is a laser, e.g., a 50-mW 785-nm diode laser. Raman spectroscopy has become a routine tool for exploring the structure and chemical properties of materials. It can provide more information than IR spectroscopy. There are three types of signal in a typical Raman experiment as illustrated in Figure 10.19. 

Most polymers can be analysed as received, as pellets, powders, films, fibres, in solution, or even as whole articles such as mouldings. Fine fibres can present some difficulties if a Raman microscope is not available. Raman spectroscopy has found applications in the identification of polymers in which additives obscure the polymer peaks in the IR spectrum. Reclaimed polymer is more prone to fluorescence than virgin material, causing problems for Raman analysis [394], Laser-Raman spectroscopy often allows polymer identification (e.g. in recycled material) only in conjunction with IR spectroscopy. Raman spectroscopy has been used to examine weathered PVC plasticised with DOP, DOA and BBP for dehydrochlorination [395], Laser-Raman spectroscopy has also been proposed as a suitable method for precise detection of ageing deterioration of vinyl chloride resins containing plasticisers and fillers used as electrical wire and cable coatings [396]. 

A wide range of spectroscopic techniques can be used with low-temperature matrices, and fairly routine spectrometers will usually suffice. Nevertheless, NMR spectroscopy is, for aU practical purposes, unavailable. A few research groups have developed special forms of NMR for matrices, but the solid-state spectra obtained are of low resolution, lacking the coupling information that makes conventional NMR in liquids such a powerful structural tool. Usually, matrix chemists make do with IR and UV-visible spectroscopy, supplemented where appropriate with less common techniques such as Raman spectroscopy, laser-induced emission, and EPR. 

Somasak Naviroj is currently a Ph.D. candidate in the Department of Macromolecular Science, Case Western Reserve University, Cleveland. He is presently studying the molecular structure of composite Interfaces using FT-IR and structure of silanes in aqueous solution by laser Raman spectroscopy. 

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in usmg Raman for the vibrational characterization of surfaces arises from the fact that the teclmique can be used in situ under non-vacuum enviromnents, and also because it follows selection rules that complement those of IR spectroscopy. 

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