Posted by Steve Simon on March 19, 2013, 3:12 am, in reply to "Inelastic scattering of photons"
The simplest answer is that there are just not a lot of values of k' that are allowed as finite states. So the total probability of scattering is small because there are very few final states to land in. In addition to this, there is another factor. In Fermi's golden rule there is an energy conserving delta function delta(E_i  E_f) Now suppose that we write E = c k then (I set hbar=1) we have delta(c (k  k') ) = delta(k  k) /c note that c is downstairs  hence the result is small. 
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