Raman spectroscopy band intensity

Because the Raman cross-section of molecules is usually low, intense light sources and low-noise detectors must be used, and high sensitivities - as required for surface analysis - are difficult to achieve. Different approaches, singly and in combination, enable the detection of Raman spectroscopy bands from surfaces. 

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 and IR spectroscopies are complementary to each other because of their different selection rules. Raman scattering occurs when the electric field of light induces a dipole moment by changing the polarizability of the molecules. In Raman spectroscopy the intensity of a band is linearly related to the concentration of the species. IR spectroscopy, on the other hand, requires an intrinsic dipole moment to exist for charge with molecular vibration. The concentration of the absorbing species is proportional to the logarithm of the ratio of the incident and transmitted intensities in the latter technique. 

Quantitative measurements are a little more involved in Raman spectroscopy. In infrared spectroscopy, the concentration of a functional group is linearly dependent on the absorbance of its related band, absorbance being the logarithmic ratio of the intensities of the incident and transmitted radiation. This means that both short and long term fluctuations in the intensity of the radiation source are irrelevant. However, in the case of Raman spectroscopy, the intensity of a band is linearly dependent on the concentration of its related functional group. This means that direct measurements are required and this is not always possible, hence ratio techniques are commonly used and, in the case of solutions, an internal standard may be added to the solution. 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

The first Raman and infrared studies on orthorhombic sulfur date back to the 1930s. The older literature has been reviewed before [78, 92-94]. Only after the normal coordinate treatment of the Sg molecule by Scott et al. [78] was it possible to improve the earlier assignments, especially of the lattice vibrations and crystal components of the intramolecular vibrations. In addition, two technical achievements stimulated the efforts in vibrational spectroscopy since late 1960s the invention of the laser as an intense monochromatic light source for Raman spectroscopy and the development of Fourier transform interferometry in infrared spectroscopy. Both techniques allowed to record vibrational spectra of higher resolution and to detect bands of lower intensity. 

Raman spectroscopy of matrix-isolated molecules carries some difficulties conneeted with the possibility of local heating of the matrix under laser irradiation. Besides, because of the relatively low intensity of Raman bands, higher concentrations of the species to be studied are needed in the matrix (the ratio of matrix gas to reagent = 100-500). As a result, the effective isolation of reactive intermediates is prevented. 

In the early 1990s Raman spectroscopy was applied to the characterization of TS-1 catalysts [55,56]. In such experiments, beside the 960 cm band, already observed by IR spectroscopy (see Sect. 3.5), a new component at 1125 cm was detected by Scarano et al. [55] (see Fig. 2f). The 1125 cm band was recognized to be a fingerprint of the insertion of Ti atoms in the ze-olitic framework [55]. This band could not be observed in the IR studies as totally overshadowed by an extremely intense band around 1000 cm due to Si02 framework modes (Fig. 2e). 

The vibrational spectrum of a metal complex is one of the most convenient and unambigious methods of characterization. However, it has not been possible to study the interactions of metal ions and biological polymers in this way since the number of vibrational bands from the polymer obscure the metal spectrum. The use of laser techniques for Raman spectroscopy now make it very likely that the Raman spectra of metals in the presence of large amounts of biological material will be measured (34). The intensity of Raman lines from metal-ligand vibrations can be... 

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

In addition to the characteristic XRD patterns and photoluminescence, UV-visible and X-ray absorption spectra, another fingerprint thought to indicate lattice substitution of titanium sites was the vibrational band at 960 cm-1, which has been recorded by infrared and Raman spectroscopy (33,34). Although there is some controversy about the origin of this band, its presence is usually characteristic of a good TS-1 catalyst, although it turned out to be experimentally extremely difficult to establish quantitative correlations between the intensity of the 960 cm-1 band and the Ti content of a Ti silicate and/or its catalytic activity. 

Some characteristics of, and comparisons between, surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRRAS) for examining reactive as well as stable electrochemical adsorbates are illustrated by means of selected recent results from our laboratory. The differences in vibrational selection rules for surface Raman and infrared spectroscopy are discussed for the case of azide adsorbed on silver, and used to distinguish between "flat" and "end-on" surface orientations. Vibrational band intensity-coverage relationships are briefly considered for some other systems that are unlikely to involve coverage-induced reorientation. 

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