Phonon Raman spectroscopy in systems with electronic degeneracy

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Abstract
The theory of the vibrational or phonon Raman spectrum of molecules or solids in the nonresonant regime is formulated for systems with electronic degeneracy. A selection rule is established, extending that of Child (1963), previous disagreement on this selection rule is reviewed. Symmetric and antisymmetric parts of the scattering amplitude are proved to combine incoherently, as observed by Kiel and co-workers (1969). Tables of Raman scattering contributions in nondegenerate systems for oriented scatterers by Loudon (1964) and for orientationally averaged scatterers by McClain (1971) are generalised to systems with electronic degeneracy. This should greatly facilitate the assignment of spectral lines in systems with Jahn-Teller or Kramers degenerate ground states, and constitutes the first step in determining a strategy for the economical extraction of the information available through Raman spectroscopy. An alternative recoupling is considered following work in quantum beat spectroscopy. This will prove advantageous when only linear polarisation is used, or when particular intermediate state symmetries are important.

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