Raman scattering excitation spectrum

One of the main advantages of Raman spectroscopy over IR is that water is a weak Raman scatterer. The spectrum of water causes little interference so that spectra of solutes can be measured in aqueous solutions. A good example of the reduced interference from water is shown for two pharmaceuticals in Fig. 7-28. The Raman spectra of damp and dry samples of acetaminophen and ibuprofen are shown in the figure. Bands due to water are not observed in the spectra. Near and mid-IR of these same samples exhibited relatively strong absorbances due to water. These Raman spectra were measured on a dispersive instrument and were excited with an Ar-ion laser emitting at 488 nm. The background for the acetaminophen sample is flat, whereas ibuprofen exhibits a background characteristic of fluorescence. 

Fi . 8c. Raman scattering excitation profiles, deduced from the intensities of the C=C-stretching modes Vi ( x), Vi (O) v j ( ), for different laser frequencies, and the absorption spectrum of a sixteen layer 6a multilayer after 245 min. UV-irradiation... 

Raman scattering spectroscopy is used to probe the vibrational excitations of a sample, by measuring the wavelength change of a scattered monochromatic light beam. This is usually performed by impinging a monochromatic laser beam to the sample surface, and by recording the scattered beam spectrum. 

Because the time scale of the Raman scattering event ( 3.3 x 10-14s for a vibration with wavenumber shift 1000 cm-1 excited in the visible) is much shorter than that of the fastest conformational fluctuations in biomolecules, the ROA spectrum is a superposition of snapshot spectra from all the distinct chiral conformers present in the sample. Together with the dependence of ROA on chirality, this leads to an enhanced sensitivity to the dynamic aspects of biomolecular structure. The two-group model provides a qualitative explanation since it predicts ROA intensities that depend on absolute chirality in the form of a sin x dependence... 

Figure 20 Raman scattering from PVC thermally degraded at 80°C. The excitation laser was 514 nm the degradation time in minutes is indicated above each spectrum. Reproduced form Everall [51]. , with kind permission of Springer Science and Business Media. 

A small fraction of the molecules are in vibrationally excited states. Raman scattering from vibrationally excited molecules leaves the molecule in the ground state. The scattered photon appears at higher energy, as shown in Figure lb. This anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-shifted spectrum, but at room temperature it is strong enough to be useful for vibrational frequencies less than about 1500 cm 1. The Stokes and anti-Stokes spectra contain the same frequency information. 

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. 

Figure 3. Resonance Raman spectrum of purple acid phosphatase. Protein (5 mM) maintained at 5 C In a glass Dewar and probed with 514.5 nm excitation (within the 560 nm phenolate + Fe(III) CT band, e = 4,000 cm" M The broad, underlying feature from 400-550 cm"1 Is due to Raman scattering from glass. (Reproduced from Ref. 14. Copyright 1987 American Chemical Society.)...

Physical Properties Density average and standard deviation Size average and standard deviation Fluorescence Fluorescence excitation spectrum Scattered light spectrum Absorption spectrum (from microwave to UV) Raman spectrum Electrical conductivity, impedance Acoustic properties... 

Figure 12.6—The diverse components of a fluorescence spectrum. The position of the Raman scattering band depends on the wavelength of excitation and the nature of the solvent.

FIGURE 5. Schematic diagram of low resolution (0.8 nm) Raman scattering spectrum of O3 excited at 266 nm. The spectrum consists of overtones and combination bands in v-] (antisymmetric stretch) and v3 (antisymmetric stretch up to v" = 7 and V3 = 6 No bands with >2 (bending) are evident, suggesting that the bond angle remains the same during the transition. Reproduced from reference (80) with permission from the American Chemical Society. 

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