Raman Characterisation of Polymer Orientation

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  

In the usual Raman experiment, the observations are made perpendicular to the direction of the incident beam, which is plane polarised. The depolarisation ratio is defined as the intensity ratio of the two polarised components of the scattered light which are parallel and perpendicular to the direction of the propagation of the (polarised) incident light. The polarisation of the incident beam is perpendicular to the plane of propagation and observation. For this geometry, the depolarisation ratio is defined as the intensity ratio  

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. 

In solids, the problem of polarisation is more complicated, but the results are more rewarding (a.30). In solids, the molecular species are oriented with respect to each other. Therefore, the molecular 

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