1. The problem statement, all variables and given/known data Discuss the difference between phonon related ir-absorption processes in Ge and GaAs. For instance, excitation of TO-phonon by photon. 2. Relevant equations 3. The attempt at a solution I really don't know how I should phrase my answer. Thoughts that I have include: Since GaAs is polar and Ge isn't, it should have an influence of the absorption process. So perhaps the process is more likely to occur in Ge? For example, you can easily observe two-phonon absorption induced by a photon (k = 0) in Ge, since there is no TO-phonon absorption. Another attempt could be to reason about the dielectric constant that is higher in Ge, 16.2, compared to GaAs, 12.9. And that the TO-phonon frequencies are higher in Ge. Am I on my way to get the solution perhaps?