The interaction between a nucleus and its surrounding environment is known as a hyperfine interaction. These interactions are very small compared to the energy levels of the nucleus itself but the extreme energy resolution of the Mössbauer effect enables these interactions to be observed. The hyperfine interactions may shift energy levels or lift their degeneracy. Both of these variations will affect the shape of a Mössbauer spectrum.
The nuclear Hamiltonian can be expressed as
These effects will be discussed in turn with reference to their physical causes and their effects on the Mössbauer spectrum lineshapes and positions.