Normal Raman spectroscopy

McCreery R L, Liu Y-C, Kagen M, Chen P and Fryling M 1996 Resonance and normal Raman spectroscopy of carbon surfaces relationships of surface structure and reactivity ICORS 96 XVth Int. Conf. on Raman Spectroscopy ed S A Asher and P B Stein (New York Wiley) pp 566-7... 

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. 

As its name implies, this technique offers very much greater sensitivity, by up to X ]06 compared with normal Raman spectroscopy. This occurs on roughened metal surfaces such as electrodes and cold-evaporated films. Unfortunately, the phenomenon at its best is highly selective, limited principally to the metals Cu, Ag, and Au. On these metals excellent spectra can be obtained (27, 28). Nevertheless, a few encouraging successes have been reported for the typical group Vlll metals, such as Ni, Pd and Pt, which are of principal catalytic interest (29). 

Mosier-Boss PA, Lieberman SH. Detection of nitrate and sulfate anions by normal Raman spectroscopy and SERS of cationic-coated, silver substrates. Applied Spectroscopy 2000, 54, 1126-1135. 

Compared with normal Raman spectroscopy, the SERS technique gives a considerably lower limit of determination of the substance to be analyzed. This is achieved by bringing colloidal noble metal atoms into close proximity to the sample molecules after they have been separated by TLC. Further information can he found in [90, 91]. 

FT-Raman Spectroscopy. In order to get information about the observed chemical changes of the surface, FT-Raman spectroscopy was used to examine the surface after laser irradiation. Due to the high penetration depth of the Nd YAG laser beam of the spectrometer we were well aware that possible information would mainly consist of bulk properties. But nevertheless the spectra are valuable because there is also a large possibility of decomposition, especially of the triazene chromophore in the bulk. Raman spectroscopy was used, as compared to IR spectroscopy, because of the less complex band structure of the polymer and the larger Raman cross section of the N=N-N chromophore. Measurements with normal Raman spectroscopy were not possible, due to a high fluorescence background with the use of visible excitation light. 

In normal Raman spectroscopy, the exciting frequency lies in the region where the compound has no electronic absorption band. In resonance Raman spectroscopy, the exciting frequency falls within the electronic band (Sec. 1.2). In the gaseous phase, this tends to cause resonance fluorescence since the rotational-vibrational levels are discrete. In the liquid and solid states, however, these levels are no longer discrete because of molecular collisions and/or intermolecular interactions. If such a broad vibronic bands is excited, it tends to give resonance Raman rather than resonance fluorescence spectra [101,102]. 

The origin of resonance Raman enhancement is explained in terms of Eq. 1.201. In normal Raman spectroscopy, Vo is chosen in the region that is far from the electronic absorption. Then, v vq, and a.p is independent of the exciting frequency vq. In resonance Raman spectroscopy, the denominator, Vq, becomes very small as vq approaches v. Thus, the first term in the square brackets of Eq. 1.201 dominates all other terms and results in striking enhancement of Raman lines. However, Eq. 1.201 cannot account for the selectivity of resonance Raman enhancement since it is not specific about the states of the molecule. Albrecht [103] derived a more specific equation for the initial and final states of resonance Raman scattering by... 

In situ spectroscopic and optical methods NRS Normal Raman Spectroscopy... 

Explain why fluorescence is a problem in normal Raman spectroscopy. Give two examples of how the fluorescence interference in Raman spectroscopy can be minimized or eliminated. 

There have been a number of recent investigations in the field of normal Raman spectroscopy with laser excitation of proteins in aqueous solutions . Raman spectroscopy uncovers information of the peptide backbone, geometry of... 

Fundamentals. Provided enough substance is present at the electrochemical interface in a fairly thick film of corrosion products or in a modifying layer, identification of the constituent matter is possible with (normal) Raman spectroscopy (NRS). Although this approach is not exactly surface specific, it is included because it has been applied frequently. The same argument applies to Raman spectroscopy applied with the help of a microscopy (Raman microscope). 

Raman scattering features most of the above-sketched properties - but not the high cross section. Quite on the contrary, Raman cross sections are about 12 orders of magnitude lower than cross sections sufficient for SMD. Hence, normal Raman spectroscopy is inadequate for SMD. 

If an enhancement of five to six orders of magnitude can be achieved routinely, TERS for small molecules, which are not in resonance with the laser line, is within reach. These molecules have cross sections of the order of (dff/dfJ) 10 cm sr and are barely seen as adlayers on smooth interfaces by normal Raman spectroscopy. To test this, benzenethiol was chosen, which assumes an essentially vertical orientation to the surface due to its thiol group. The adlayer preparation is quick and easy the adsorption occurs from an etha-nolic solution, and a self-assembled monolayer is formed on previously flame-annealed gold or platinum surfaces. Fig. 10.26 shows TERS spectra for benzenethiol at Au(llO) and Pt(llO) surfaces. The comparison of the spectra reveals the characteristic benzenethiol bands but with sHghtly different band positions and relative intensities for the two samples and a nearly 20-fold lower intensity level for benzenethiol at Pt(llO). The comparison of these data with SER spectra for... 

One can compare these results with those obtained by SERS (Surface Enhanced Raman Scattering). This technique is more sensitive than normal Raman spectroscopy to detect a particular species, leading a lowering of the limit of detection. However the implementation of this method can be more... 

The enhancement factor is a measure which allows a quantification of the signal increase when using SERS as compared to normal Raman spectroscopy. There are several different issues that can or have to be considered when determining an enhancement factor. Therefore, different kinds of enhancement factors, respectively, and different modes of calculations can be found in the literature. Three common enhancement factors for SERS are the single-molecule enhancement factor (SMEF), an enhancement factor considering SERS substrates... 

Normal Raman spectroscopy probes the variations of the polarizability tensor with respect to the degrees of freedom, in the ground electronic state. When an electrical field is applied to a system the electron distribution is modified and the sample acquires an induced dipole moment as the barycenters of the charges are displaced. The polarizability tensor [a] defines the correspondence between the incident electrical field E and the induced dipole moment M = [a]E. The polarizability tensor can be expanded in a Taylor series analogous to Equation (8.8) ... 

In normal Raman spectroscopy a sample is placed in a (monochromatic) laser beam and the very weak scattered light of lower frequency is studied. In such a study the colour of the laser light is usually chosen to be away from any absorption band of the sample because such a choice reduces the risk that the focused laser beam will destroy the sample by heating it. In the resonance Raman effect the laser beam colour is deliberately chosen to coincide with an absorption band—an electronic transition—of the sample. Whilst this may lead to the destruction of the sample, for favourable cases it leads to Raman scattering which is much stronger than normal. This, in turn, means that the laser power can be reduced, improving the chances of sample survival. The spectra obtained from compounds showing such a resonance Raman effect are both simpler and more complicated than normal Raman spectra. They are simpler because, often, only totally symmetric vibrational modes are seen. The reason for this is that if the electronic... 

Table 1 Environmentally Important Compounds Studied by Normal Raman Spectroscopy (Dispersive and FT), Resonance Raman Spectroscopy (Including Preresonance), and Surface-Enhanced Raman Spectroscopy... 

Figure 1 Number of journal publications devoted to environmental applications of various Raman techniques as a function of years. The resurgence of normal Raman spectroscopy in the late 1990s due to the improvements in Raman instrumentation is obvious. Also, a recent increase in SERS applications can clearly be seen. Moreover, the maturity of Raman spectroscopy as an important analytical tool in the analysis of complex environmental mixtures is seen in the steady growth of hyphenated techniques involving Raman spectroscopy. Note that publications reporting the use of hyphenated techniques were included in the tally for both hyphenated techniques as well as the specific Raman method employed (i.e., NRS, RRS, and SERS). In addition, publications reporting the use of the SERRS technique were included in the tally for both RRS and SERS. Hence, the Total category is not always equal to the sum of the NRS, RRS, and SERS applications.

Normal Raman spectroscopy has also been coupled to separation techniques such as CE [36,37,44] and TLC [42]. Morris and co-workers have shown CE-NRS to be an attractive analytical tool due to the short analysis times, relative ease in experiment setup, the high information content given by Raman, and, perhaps most importantly, the electrophoretic preconcentration enhancement factors that lower detection limits of normal Raman [44]. Two general classes of electrophoretic methods of concentration can be per-... 

Figure 7 Capillary electrophoresis-NRS analysis of 5 X 10 " M solution of nitrate (31 ppm) and perchlorate (50 ppm) at 1-sec integration. The spectrum has been ratioed to the background electrolyte spectrum and smoothed by a five-point Savisky-Golay function. With electrophoretic pre-concentration by sample stacking, 620 ppb and 1 ppm detection limits are obtained for nitrate and perchlorate ions, respectively. (Reprinted with permission from WK Kowalchyk, PA Walker, MD Morris. Rapid normal Raman-spectroscopy of sub-ppm oxy-anion solutions The role of electrophoretic preconcentration. Appl Spectrosc 49 1183-1188, 1995. Copyright 1995 Society for Applied Spectroscopy.)...

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