ABSTRACT
The physical properties of condensed matter often result from an intricate interplay between the electronic, nuclear, orbital, and spin degrees of freedom. This is in particular also true for the transient processes occurring following photoexcitation of materials. Excitation using photons in the visible or ultraviolet part of the electromagnetic spectrum typically leads to the creation of highly excited electron-hole pairs, which on a very fast time scale thermalize among themselves. After these thermalization processes, kinetic energy relaxation takes place through energy transfer to other fundamental excitations of the system. A simple example of this is photoexcitation in semiconductors, where the electron-hole energy relaxation typically takes place through Fröhlich interaction-mediated excitation of longitudinal optical phonons (Chapter 8). Lattice anharmonicity leads to a subsequent decay of the longitudinal optical phonons into lower-energy phonons belonging, for instance, to the acoustical branches.
