Principles of Raman spectroscopy

Remember that linear (non-linear) A-atomic molecules have 3N — 5 (3N — 6) so-called normal modes 

Raman shifting (rotational or ro-vibrational) can only occur if the molecule s polarizability a changes during its vibrational or rotational motion along its normal coordinates, i.e. 

An electric field (here associated with the laser radiation field) induces a dipole moment given by 

The absolute intensity of the Raman scattered light can be described mathematically, to a good approximation, by using the equations of Placzek s theory (see Box 8.1). Because it is actually possible to calculate the Raman line intensities, and compare them with observations, Raman spectroscopy can be (and 

The essential assumption of Placzek s theory for Raman line intensities is that the frequency of the exciting radiation vl is larger than the frequency associated with the energy difference between two ro-vibrational states Vvib,rot but is less than the frequency associated with an electronically excited energy level Vei- In general, this is easy to realize in the majority of Raman scattering experiments. For the Stokes and anti-Stokes line intensities and one finds 

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For a detailed description of the basic principles of Raman spectroscopy, the associated instrumentation and potential for spectroscopic imaging, the reader is referred to some of the many excellent texts in the literature. This chapter provides an introduction to Raman spectroscopy and how it is measured. It outlines some experimental considerations specific to biospectroscopy and explores applications from molecular through cellular to tissue imaging for biochemical analysis and disease diagnostics. The complementarities and potential advantages over IR spectroscopy [Fourier Transform (FTIR) and Synchrotron Fourier Transform (S-FTIR)] are described. Finally, the future potential of the development of Raman spectroscopy for biochemical analysis and in vivo disease diagnostics are projected. 

The signal generation principle of Raman is inelastic molecular light scattering, in contrast to resonant energy absorption/emission in IR spectroscopy. During the measurement, the sample is irradiated with intense monochromatic radiation. While most of this radiation is transmitted, refracted or reflected, a small amount is scattered at the molecules. 

The measurement of vibrational optical activity requires the optimization of signal quality, since the experimental intensities are between three and six orders of magnitude smaller than the parent IR absorption or Raman scattering intensities. To date all successful measurements have employed the principles of modulation spectroscopy so as to overcome short-term instabilities and noise and thereby to measure VOA intensities accurately. In this approach, the polarization of the incident radiation is modulated between left and tight circular states and the difference intensity, averaged over many modulation cycles, is retained. In spite of this common basis, there are major differences in measurement technique and instrumentation between VCD and ROA consequently, the basic experimental methodology of these two techniques will be described separately. 

FIGURE 2 Principle of fiber bundles. (Adapted from A. D. Gift, J. Ma, K. S. Haber, B. L. McClain, and D. Ben-Amotz, Journal of Raman Spectroscopy, 30, 757-765,1999.)... 

A variety of other excellent texts are available for in-depth review of the fundamentals of Raman spectroscopy, including core technologies and applications [2, 3]. This is intended as a very brief, non-rigorous overview for non-spectroscopists who may be unfamiliar with the principles of Raman, its strengths, and practical limitations. For discussion of the experimental details of variant techniques such as ROA (Raman optical activity) or SERS, the reader is directed to the appropriate chapters in this text. 

Edwards, El. G. M. (2004). Forensic applications of Raman spectroscopy to the nondestructive analysis of biomaterials and their degradation, in Forensic Geoscience Principles, Techniques and Applications (K. Pye and D. J. Croft, Eds.). London Geological Society Special Publication 232,159-170. 

Thus far, we have reviewed basic theories and experimental techniques of Raman spectroscopy. In this chapter we shall discuss the principles, experimental design and typical applications of Raman spectroscopy that require special treatments. These include high pressure Raman spectroscopy, Raman microscopy, surface-enhanced Raman spectroscopy, Raman spectroelectro-chemistry, time-resolved Raman spectroscopy, matrix-isolation Raman spectroscopy, two-dimensional correlation Raman spectroscopy, Raman imaging spectrometry and non-linear Raman spectroscopy. The applications of Raman spectroscopy discussed in this chapter are brief in nature and are shown to illustrate the various techniques. Later chapters are devoted to a more extensive discussion of Raman applications to indicate the breadth and usefulness of the Raman technique. 

In this paper we will present some examples of the application of resonance Raman spectroscopy to the study of transition metal diatomics. The application of Raman spectroscopy to matrix-isolated metal clusters was first reported by Schulze et al. (] ). Having observed only a single line in the Raman spectrum of Ag3, Schulze concluded that the molecule was linear since a bent triatomic and an equilateral triangular geometry would have, in principle, 3 and 2 Raman-active modes. The evidence, however, is not conclusive since many Czy molecules have very weak asymmetric stretches in the Raman ( ) (for example, the V3 mode of O3 is undetectable in the Raman (3 )). Moreover, the bend (V2) of Ag3 is expected to be a very low-frequency mode, perhaps lower than one can feasibly detect in a matrix Raman experiment. 

D. A. Long, Raman Spectroscopy , McGraw-Hill, New York, 1977. The most comprehensive, unified and fully illustrated treatment of the basic theory and physical principles of Raman, resonance Raman and nonlinear Raman scattering. Readers will be inspired by the elegance and information content of the technique to delve further into the book. 

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). 

The principles, scope and limitations of Raman spectroscopy and its application in the carotenoid field has been discussed [134], Time-resolved resonance Raman spectroscopy provides useful information on carotenoids in the excited state [135]. Differences between the resonance Raman spectra of a neutral carotenoid and its cation radical provide structural information on the cation radical. 

This article first briefly surveys the history of Raman spectroscopy applied to electrochemistry, followed by a brief outline of the basic principles of surface Raman spectroscopy. The SERS phenomena and mechanisms are then introduced. This is followed by a detailed description of Raman instrumentation, and the... 

Hochleitner, R., Tarcea, N., Simon, G. et al. (2004) Micro-Raman spectroscopy a valuable tool for the investigation of extraterrestrial material. Journal of Raman Spectroscopy, 35, 515-518. (15) Choo-Smith, L.P., Edwards, H.G., Endtz, H.P. et al. (2002) Medical apphcations of Raman spectroscopy from proof of principle to clinical implementation. Biopolymers, 67,1-9. 

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