What is the momentum of an electron?

[iVenky]

I was learning about indirect band gap and it says that when the electron comes down from upper energy level to lower energy level it emits photon(which I can understand) but in order to conserve the momentum it emits phonons (which I couldn't understand).

Can you please help me with this?

Thanks a lot.

 

[Naty1]

Try wikipedia here:

http://en.wikipedia.org/wiki/Indirect_band_gap

Note the diagrams near the top of the page...

I don't know much about phonon quasi particles except that they are associated with vibrating structures like lattices...

In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice.

Is this your situation??...read further along in the wiki article and note

...photons cannot carry crystal momentum,...in an indirect band gap material, the process must also involve the absorption or emission of a phonon, where the phonon momentum equals the difference between the electron and hole momentum.
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