Principles of Raman Scattering

It is convenient to plot the Raman spectmm as intensity vs. shift in wavenumbers in cm because these can be related directly to IR spectra. The Raman shift in cm is identical to the IR absorption peak in cm for a given vibration, because both processes are exciting the same vibration. The Raman spectrum for benzene is shown in Fig. 4.62, along with the related IR transmission spectrum. 

Less commonly, the molecule decreases in vibrational energy after interacting with a photon. This might occur if the molecule is in an excited vibrational state to begin with 

---

One of the best available texts describing the principles of Raman scattering from crystals. Includes factor group calculations, polarization measurements, force constant calculations, and many other aspects of crystal physics. 

Figure 19 Schematic representation of the Raman effect (a) principle of Raman scattering, if/ is the scattering angle (b) vector representation of the momentum conservation rule.

The general principles of Raman scattering of glass are based essentially on two kinds of consideration ... 

Whatever the case, the physical basic principle of Raman scattering is illustrated by Fig. 20 see Ref. 28. 

If the resolving capacity of the instruments is ideal then vibrational-rotational absorption and Raman spectra make it possible in principle to divide and study separately vibrational and orientational relaxation of molecules in gases and liquids. First one transforms the observed spectrum of infrared absorption FIR and that of Raman scattering FR into spectral functions... 

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. 

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. 

The theory of the method is rather complicated, because the amplitude of Raman scattering is described by a second-rank tensor, so it will not be discussed here. Just like for fluorescence, P2 cosff)) and (P4(cosd)), or cos 6) and (cos" 6), can, at least in principle, be obtained for the simplest type of uniaxial orientation distribution and these values now refer directly to the molecules of the polymer itself. In practice it is often necessary to make various simplifying assumptions. For biaxially oriented samples several other averages can be obtained. 

Fig. 28 Illustration of the principle of selective imaging by use of specific spectral bands of Raman scattered radiation. (Reproduced with permission from G. J. Rosasco, Raman Microscope Spectroscopy, in Advances in infrared and Raman Spectroscopy, Vol. 7. Copyright 1980, Heyden Son Ltd.)...

In principle, resonance Raman scattering allows us to observe and study each vibrational mode separately, provided it is active. The activity of totally symmetric modes is basically governed by the displacement param-... 

Let us concentrate first on the G peak because of the linear electronic dispersion of graphene, in principle the Raman scattering process of the G peak can be always resonant, i.e. the electronic transition can always match with real electronic states, no matter the incident excitation frequency. This is described in Fig. 4a in the case of incident resonance (scattered resonance is also possible). In general, a resonant process has a higher probability compared to a non-resonant process, so one would expect the G peak to be described only by resonant processes. However, it has been shown, both experimentally and theoretically, that quantum interference effects cancel the resonant contributions in the G peak Raman intensity. Consequently,... 

To the extent that we can trust the harmonic approximation, each level of vibrational excitation (each increment in vibrational quantum number v) costs one vibrational constant in energy. As Table 8.2 shows, the vibrational constants of typical stretching motions place vibrational excitation energies in the infrared region of the spectrum. We can measure vibrational transitions that occur by absorption or emission or by scattering. Section 6.3 introduced the concept of Raman scattering, which in principle can be applied to the spectroscopy of any degree of freedom, but which is most commonly used for spectroscopy of vibrational states. 

The basic principles of Raman SNOM have been demonstrated [34-46]. The considerable potential of the technique, however, has yet to be fully realized. The key limitation is the very low-excitation intensity afforded by the probes currently available. This results in lengthy, difficult experiments that often generate only weak, low-signal-to-noise Raman spectra. As a result, RSNOM is currently generally confined to the study of relatively strong Raman scatterers with moderately large aperture probes (—100-200 nm). 

Principles and Characteristics Raman microscopy takes advantage of the fact that the intensity of Raman scattered light is independent of sample volume [503]. Thus, the light intensity remains essentially constant with decreasing sample size down to the dimension determined by the diffraction limit, and hence the wavelength, of the laser excitation. Raman intensity always comes from... 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

Similarly, the first-order expansion of the p° and a of Eq. (5.1) is, respectively, responsible for IR absorption and Raman scattering. According to the parity, one can easily understand that selection mles for hyper-Raman scattering are rather similar to those for IR [17,18]. Moreover, some of the silent modes, which are IR- and Raman-inactive vibrational modes, can be allowed in hyper-Raman scattering because of the nonlinearity. Incidentally, hyper-Raman-active modes and Raman-active modes are mutually exclusive in centrosymmetric molecules. Similar to Raman spectroscopy, hyper-Raman spectroscopy is feasible by visible excitation. Therefore, hyper-Raman spectroscopy can, in principle, be used as an alternative for IR spectroscopy, especially in IR-opaque media such as an aqueous solution [103]. Moreover, its spatial resolution, caused by the diffraction limit, is expected to be much better than IR microscopy. 

Principles and Characteristics The prospects of Raman analysis for structural information depend upon many factors, including sample scattering strength, concentration, stability, fluorescence and background scattering/fluorescence from the TLC substrate. Conventional dispersive Raman spectroscopy has been considered as a tool for in situ analysis of TLC spots, since most adsorbents give weak Raman spectra and minimal interference with the spectra of the adsorbed species. Usually both silica and cellulose plates yield good-quality conventional Raman spectra, as opposed to polyamide plates. Detection limits for TLC fractions... 

Raman scattering is a linear spectroscopy, in principle, meaning that the Raman scattering intensity, 7S, scales linearly with the intensity of the incident light, IL, provided the scattering compound can be considered as optically thin. At fixed incident light intensity IL, the Raman response scales with the population density of the scatterers, N(E) according to... 

In contrast to Raman scattering, the absorption of infrared (IR) radiation is a first-order process, and in principle a surface or an interface can generate a sufficiently strong signal to yield good IR spectra [6]. However, most solvents, in particular water, absorb strongly in the infrared. There is no special surface enhancement effect, and the signal from the interface must be separated from that of the bulk of the solution. 

Much of this research effort has been directed to the study of the fundamental basis of surface-enhanced Raman scattering (SERS), in order to understand the underlying principles. There have also been many applications of SERS to situations in which in situ vibrational... 

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. 

Zheltikov, A. M. 2000. Coherent anti-8tokes Raman scattering From proof-of-the-principle experiments to femtosecond CAR8 and higher order wave-mixing generalizations. J. Raman Spectrosc. 31(8-9) 653-67. 

Ab initio and semiempirical molecular orbital (MO) model calculations have become an efficient way to predict chemical structures and vibrational (i.e., Raman scattering and IR emission) spectra. We and others have used such approaches to better understand certain features of fhe specfra, as explained in the following. The basic principles underlying ab initio model calculations have been described in many textbooks and papers (see for example Refs. 44,47,48). Applications in relation to ILs and similar systems have also been reported, as discussed later. 

---