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10. Appendix

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The diagram for the process labeled (2) is:<br />

Solution to Problem 7.8 655<br />

These diagrams translate into perturbation terms similar to that in (6.61).<br />

In both of the above diagrams the vertex for the electron-photon interaction<br />

occurs first. We can generate other diagrams in which the electron(hole)phonon<br />

interaction occurs first. For example, the following is a diagram in<br />

which the electron-phonon interaction excites an electron from an intermediate<br />

valence band (°v ′) to the conduction band (¢c) and then the photon<br />

excites another electron from the final valence band state (°v) to fill the hole<br />

in °v ′.<br />

This diagram can also be translated into an expression similar to (6.61). However,<br />

since the electron-hole pair has to be created via emission of a phonon<br />

the energy denominator in this case has the form: Ecv ′ ˆphonon and this term<br />

is highly non-resonant.<br />

(c) The Feynman diagrams for two-phonon Raman scattering processes can be<br />

divided into three types. These are labeled (a) to (c) in the following figure.

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