Inverse Raman effect
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If a material is simultaneously irradiated by intense monochromatic light of frequency νL (typically a laser beam) and light of a continuum of higher frequencies, among the possibilities for light scattering are scattering:
- from the monochromatic beam at νL to the continuum at νL+νM (anti-Stokes Raman scattering)
- from the continuum at νL+νM to the monochromatic beam at νL (Stokes Raman scattering)
where νM is a Raman frequency of the material. The strength of these two scatterings depends (among other things) on the energy levels of the material, their occupancy, and the intensity of the continuum. In some circumstances Stokes scattering can exceed anti-Stokes scattering; in these cases the continuum (on leaving the material) is observed to have an absorption line (a dip in intensity) at νL+νM. This phenomenon is referred to as the inverse Raman effect; the application of the phenomenon is referred to as inverse Raman spectroscopy, and a record of the continuum is referred to as an inverse Raman spectrum.
In the original description of the inverse Raman effect, the authors discuss both absorption from a continuum of higher frequencies and absorption from a continuum of lower frequencies. They note that absorption from a continuum of lower frequencies will not be observed if the Raman frequency of the material is vibrational in origin and if the material is in thermal equilibrium.
- W.J.Jones and B.P. Stoicheff, ‘Inverse Raman Spectra: Induced Absorption at Optical Frequencies’, Phys. Rev. Lett. 13, 657 - 659 (1964).
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