Raman spectroscopy and the phonon confined in the Brillouin zone

[phzrosmary]

Hello everyone,

I need some confirmation on something:

As far as I understood, the raman spectroscopy measures the inelastic scattering of a photon in a medium through the absorption or the emission of a phonon in the medium. The energy and the momentum is conserved.

hk_in=hk_out +- hq (1) Momentum conservation

k_in and k_out being the incoming and the scattered wave vector, and q the impuls of the phonon.
q is quantified.

now the difficulty:

why are the phonon taking part in the raman scattering limited to the central region of the first brillouin zone?

what I think:

the incoming photon has a very small wave vector compared to the dimension of the brillouin zone. q is limited to the central region of zone, otherwise q would be bigger than k_in, and that would violate (1). Is it valid? When would you use the quantified phonon as an argument for the confinement of q in the brillouin zone though?

Thank you very much for your help in advance

peace

 

[SpinFlop]

Raman is effectively a q=0 probe. The photon cannot impart enough momentum to study phonon processes along the full dispersion curve, you need inelastic x-ray or neutron scattering to do that.