# IR Spectoscopy - Photon Energies

1. May 27, 2015

### Wminus

Hi! I would like to ask some general question about IR spectroscopy.

During absorption of IR photons two quantities have to be conserved, energy $\hbar \omega_{photon} = \hbar \omega_{phonon}$ and momentum $\hbar k_{photon} = \hbar k_{phonon}$.

Wouldn't these two quantities be conserved at the same time very rarely? I mean, what's the chances of them matching perfectly?? And what if the phonon bandgap is larger than the photons in the IR bandwidth, or much smaller?

And is it true that one can use IR spectroscopy to map out the optical modes of a material?

thanks for all replies :)

2. May 28, 2015

### Wminus

Are my questions unclear, perhaps?