Experimental Raman spectroscopy

There are tluee very important sources of up-to-date infonnation on all aspects of Raman spectroscopy. Although papers dealing with Raman spectroscopy have appeared and will continue to appear in nearly every major chemical physics-physical chemistry based serial. The Journal of Raman Spectroscopy [35] is solely devoted to all aspects, both theoretical and experimental, of Raman spectroscopy. It originated in 1973 and continues to be a constant source of mfonuation on modem applications of Raman spectroscopy. 

The vibrational states of a molecule are observed experimentally via infrared and Raman spectroscopy. These techniques can help to determine molecular structure and environment. In order to gain such useful information, it is necessary to determine what vibrational motion corresponds to each peak in the spectrum. This assignment can be quite difficult due to the large number of closely spaced peaks possible even in fairly simple molecules. In order to aid in this assignment, many workers use computer simulations to calculate the vibrational frequencies of molecules. This chapter presents a brief description of the various computational techniques available. 

Figure 5.13 Experimental arrangement for gas phase Raman spectroscopy.

Experimental values of AG ii2 for high values of v are not normally obtainable from infrared or Raman spectroscopy because of the low intensities of Av = 2, 3,... 

From 1960 onwards, fhe increasing availabilify of intense, monochromatic laser sources provided a fremendous impetus to a wide range of spectroscopic investigations. The most immediately obvious application of early, essentially non-tunable, lasers was to all types of Raman spectroscopy in the gas, liquid or solid phase. The experimental techniques. 

Stimulated Raman spectroscopy is experimentally different from normal Raman spectroscopy in that the scattering is observed in the forward direction, emerging from the sample in the same direction as that of the emerging exciting radiation, or at a very small angle to it. 

Raman spectroscopy of graphite can be an experimental challenge, because the material is a strong blackbody absorber. Generally, low (1—10-mW) laser power is used to minimise heating, which causes the band positions to change. In addition, the expansion of the graphite causes the material to go out of the focus of the optical system, an effect which can be more pronounced in microprobe work. 

Normal mode analysis exists as one of the two main simulation techniques used to probe the large-scale internal dynamics of biological molecules. It has a direct connection to the experimental techniques of infrared and Raman spectroscopy, and the process of comparing these experimental results with the results of normal mode analysis continues. However, these experimental techniques are not yet able to access directly the lowest frequency modes of motion that are thought to relate to the functional motions in proteins or other large biological molecules. It is these modes, with frequencies of the order of 1 cm , that mainly concern this chapter. 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

Raman spectroscopy allowing the determination of global variation of the ionization of the membrane as a function of pH was studied. Experimental and theoretical Pka value was found to be 5.2 from which the average number of five carboxylic groups/graft was determined [145]. 

Recent developments in the mechanisms of corrosion inhibition have been discussed in reviews dealing with acid solutions " and neutral solu-tions - . Novel and improved experimental techniques, e.g. surface enhanced Raman spectroscopy , infrared spectroscopy. Auger electron spectroscopyX-ray photoelectron spectroscopyand a.c. impedance analysis have been used to study the adsorption, interaction and reaction of inhibitors at metal surfaces. 

Laser Raman spectroscopy as it is applied to the study of surface adsorbed.species involves a number of experimental problems such as fluorescence, weak Raman lines, and interfering plasma lines. Techniques of overcoming these problems have been continually improved and good... 

These models are designed to define the complex entrance effects and convection phenomena that occur in a reactor and solve the complete equations of heat, mass balance, and momentum. They can be used to optimize the design parameters of a CVD reactor such as susceptor geometry, tilt angle, flow rates, and others. To obtain a complete and thorough analysis, these models should be complemented with experimental observations, such as the flow patterns mentioned above and in situ diagnostic, such as laser Raman spectroscopy. 

A nano-light-source generated on the metallic nano-tip induces a variety of optical phenomena in a nano-volume. Hence, nano-analysis, nano-identification and nanoimaging are achieved by combining the near-field technique with many kinds of spectroscopy. The use of a metallic nano-tip applied to nanoscale spectroscopy, for example, Raman spectroscopy [9], two-photon fluorescence spectroscopy [13] and infrared absorption spectroscopy [14], was reported in 1999. We have incorporated Raman spectroscopy with tip-enhanced near-field microscopy for the direct observation of molecules. In this section, we will give a brief introduction to Raman spectroscopy and demonstrate our experimental nano-Raman spectroscopy and imaging results. Furthermore, we will describe the improvement of spatial resolution... 

Leopold et al. and Nyholm et al. have investigated this oscillatory system by in situ confocal Raman spectroscopy [43], and in situ electrochemical quartz crystal microbalance [44], and in situ pH measurement [45] with the focus being on darification of the osdllation mechanism. Based on the experimental results, a mechanism for the oscillations was proposed, in which variations in local pH close to the electrode surface play an essential role. Cu is deposited at the lower potentials ofthe oscillation followed by a simultaneous increase in pH close to the surface due to the protonation... 

The vibrational modes of the LS and HS isomers of the SCO complex [Fe (phen)2(NCS)2l (phen = 1,10-phenanthroline) have been measured by NIS (Fig. 9.38a), IR- and Raman-spectroscopy, and the vibrational frequencies and normal modes were calculated by DFT methods [44]. The calculated difference ASvib = 57-70 J moP depending on the method) is in qualitative agreement with the experimentally derived values (20-36 J mol K ). 

In summary, the combined experimental (NIS, IR- and Raman-spectroscopy) and computational (DFT) approach has enabled the identification of the vibrational modes that contribute most to the entropic driving force for SCO transition. 

The thickness of the ordered crystalline regions, termed crystallite or lamellar thickness (Lc), is an important parameter for correlations with thermodynamic and physical properties. Lc and the distribution of lamellar thicknesses can be determined by different experimental methods, including thin-section TEM mentioned earlier, atomic force microscopy, small-angle X-ray scattering and analysis of the LAM in Raman spectroscopy. 

The use of organic polymers as conductors and semiconductors in the electronics industry has led to a huge research effort in poly(thiophenes), with a focus on the modification of their electronic properties so that they can behave as both hole and electron conductors. Casado and co-workers [60] have performed combined experimental and theoretical research using Raman spectroscopy on a variety of fluorinated molecules based on oligomers of thiophene, an example of one is shown in Figure 7. 

Most chemists tend to think of infrared (IR) spectroscopy as the only form of vibrational analysis for a molecular entity. In this framework, IR is typically used as an identification assay for various intermediates and final bulk drug products, and also as a quantitative technique for solution-phase studies. Full vibrational analysis of a molecule must also include Raman spectroscopy. Although IR and Raman spectroscopy are complementary techniques, widespread use of the Raman technique in pharmaceutical investigations has been limited. Before the advent of Fourier transform techniques and lasers, experimental difficulties limited the use of Raman spectroscopy. Over the last 20 years a renaissance of the Raman technique has been seen, however, due mainly to instrumentation development. 

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