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Brillouin scattering is caused by an interaction between light and lattice phonon modes. Raman scattering is caused by an interaction between light and molecular vibrations. The key difference is that phonon modes are a collective, long-range phenomenon involving billions or more atoms, whereas molecular vibrations are localized vibrations of a single ...


In Raman it should say "optical phonon" instead of vibrational oscillation. I'm not sure if you can call the vibrations in a molecule phonons, since phonons are the vibrations of the lattice in a solid (even when the mathematical description, in first approximation are the same).


Here we go : Lets considere the lagrangian density of our free membrane. For convenience, we will use a cartesian set of coordinates, so that : $$\mathcal{L}=\frac{\rho}{2}\dot{h}-\frac{\kappa}{2}\left[{\partial_x}^2 h+{\partial_y}^2 h\right]^2$$ Apparently, this lagrangian depends on a quite large number of variables : ...


Yes, and no. Since the group of rotations is not a continuous group in real crystals, it is not possible to define spin in a meaningful way. It is only in an isotropic ideal medium that is possible to define spin for a phonon (quantized accoustic wave). Equivalently it is only possible to define a spin if the the wavelength of the phonon is long or if one ...

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