Raman scattering polarisation

An illustrative example of the influence of symmetry on the number of vibrations is provided by the homologous series of cyclic sulfur allotropes S (n = 6-12). The IR absorptions for these ring systems are weak owing to the low polarity of S-S bonds. However, sulfur is a good Raman scatterer because S-S bonds are readily polarised. The various sulfur allotropes have different symmetries in addition to a different number of sulfur atoms and, consequently, each allotrope exhibits a characteristic Raman spectrum (Figure 3.8). ... 

Fig. 1. Experimental set up (90° collection optics) for the measurement of laser Raman scattering, illustrating the definition of the depolarisation ratio. ly and Ij are the intensities of light scattered, respectively, parallel and perpendicular to the polarisation of the incident exciting beam...

In the second case above, since the states have the same symmetry, both are required to give electric-dipole-allowed transitions. The most important examples of this type, in the context of resonance Raman scattering, are the much studied metal-lo-porphyrin molecules which constitute the active sites of the haem proteins, notably haemoglobin and cytochrome c (Section 4.8). The visible and near ultraviolet absorption spectra of these systems show two -n - -n transitions of the porphyrin ring both are allowed with in-plane polarisation and excited-state symmetry. The lower... 

SEMPA Scanning Electron Microscopy with Polarisation Analysis, 37 SERS Surface Enhanced Raman Scattering, 32 SEW Surface Electromagnetic Waves Spectroscopy, 40 SEXAFS Surface EXAFS, 49... 

The first treatment of the theory of the intensities and polarisation effects to be expected in the Raman scattering from an oriented polymer sample appears to be that given by Cornell and Koenig." This treatment must be regarded at best as a very rough approximation, since the tensor nature of the effect is not taken into account properly. Snyder has given a correct account of the theory for rather special distributions of orientations of the Raman scatterers but his work concentrates on the information that can be obtained about the Raman tensors if the orientation distribution is known. The only treatment that has considered how much information can in principle be obtained about the distribution of orientations and what measurements are necessary to obtain it is that of Bower and Bower and Purvis. " In this treatment the similarities and differences between the theories of the fluorescence and Raman methods are apparent and an account of it follows. 

Fig. 8. Intensity of Raman scattering from a disc of miaxially oriented polymethylmethacrylate as a function of the orientation of the sample with respect to the polarisation vectors of the incident and scattered radiation. Results for the 604 cm line from a sample of birefringence /"nj —ri2 J (Reproduced by...

When the incident radiation interacts with the surface, it causes the free electrons to oscillate with the incident electric field and polarises the noble metal particles. This creates a strong local electric field at the particle surface known as a surface plasmon. When a molecule is in close proximity to a noble metal particle the molecule is polarised by the electric field of the noble metal particle. This leads to an enhancement of the Raman signal because the Raman scattering is proportional to the square of the local electric field. 

Raman scattering arises from the interaction between radiation induced oscillating electric dipoles and molecular vibrational modes. In general, the induced polarisability is not necessarily in the direction of the incident beam, and they are related by a second range tensor, a, thus ... 

Consequently, the Raman scattered light emanating from even a random sample is polarised to a greater or lesser extent. For randomly oriented systems, the polarisation properties are determined by the two tensor invariants of the polarisation tensor, i.e., the trace and the anisotropy. The depolarisation ratio is always less than or equal to 3/4. For a specific scattering geometry, this polarisation is dependent upon the symmetry of the molecular vibration giving rise to the line. 

However, samples must be of good optical quality to avoid polarisation scrambling, and both the laser and the Raman scatter must be polarised parallel to sample optical axes if complex corrections are to be avoided [264, 266]. Practical details have been given for measuring accurate polarised Raman intensities [272]. 

The information provided by the Raman spectrum of an oriented polymer differs from its infrared counterpart because of the fundamentally different processes involved in the generation of the spectra. In the infrared absorption process, as already noted, the absorption intensity is dependent on the angle between the electric vector and the direction of the dipole moment change. The Raman spectrum results from inelastic photon scattering details of which are determined by changes in the polarizability of the chemical bonds involved. Polarizability is a tensor quantity, which results in complications but, in principle, provides additional information. As we have seen, infi ared spectroscopy involves only one beam of polarized radiation, and the fraction of the nufotion absorbed by a molecule depends only on the orientation of the molecule with respect to the polarisation vector of the radiation. However, Raman scattering involves two beams of radiation, those of illumination and collection, and the scattered intensity depends on the orientation of the molecule with respect to the polarisation vectors of both beams, whidi may, of course, be different. This necessitates more detailed measurements in order to obtain the relevant information. 

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