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I've seen Raman spectra, such as here, where peaks at certain wavenumbers were identified with phonon vibration modes (TO, LO, TA, LA). What is the relationship behind this, and how could I make such associations? Would the process be different for Raman Spectra involving different types of silicon such as crystalline silicon, amorphous silicon, polycrystalline silicon, etc?

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Raman process can be described as inelastic light scattering - that is, part of the incident photon energy goes into internal degrees of freedom of the scattering medium. By looking at the energy spectrum of scattered photons one can deduce energies of these internal degrees - in your case these are phonon vibration modes.

Identifying modes - i.e. saying whether a peak is TO, LO or some other mode - is not straightforward and usually requires prior knowledge (maybe, someone measured it before) or ab initio calculations.

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  • $\begingroup$ Is there any way to gain this prior knowledge by studying certain topics? Or is it strictly found through first hand records of initial calculations? Also, can these phonon vibration modes associated with the peaks explain the existence of certain micro-crystals within a sample? I am getting the impression that the energy of scattered photons at certain wavenumbers in a spectra is unique to certain materials? Thanks! $\endgroup$
    – p14955
    Mar 23 '15 at 22:52
  • $\begingroup$ The vibration modes depend on the crystal structure so yes, the Raman spectrum is like a fingerprint of a material. I don't think that you can predict exact energies beforehand, but you should be able to tell from the crystal structure, which modes would be visible in Raman spectrum and which not. I know that it is possible for individual molecules. $\endgroup$
    – gigacyan
    Mar 24 '15 at 7:36

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