Mössbauer spectroscopy is a nuclear resonance technique. This means that a gamma quantum emitted from a nucleus in an excited stated is absorbed by another nucleus (from the same isotope) in the ground state.

When an excited nucleus of a free atom returns to its ground state by emitting a gamma-quantum, it acquires a recoil energy due to conservation of momentum. Therefore the energy of the emitted gamma-quantum will be to small to bring an absorbing nucleus from the ground state to its excited state. So nuclear resonance will not occur.

Rudolph L. Mössbauer in 1958 discovered that when the emitting nucleus is bound into a lattice there is a chance that the nucleus experiences no recoil. This is the case when the recoil-energy is smaller than the required energy for phonon creation. It is this recoilless emission which is called the Mössbauer effect.

Nuclear resonance will only occur when the energy of the gamma-quantum and the transition energy of the absorbing nucleus are the same. These energies are, however, affected by the hyperfine interactions between the nucleus and electrons. Fortunately, the differences in these energies are very small. The energy of the emitted gamma-quantum can be tuned to the energy levels of the absorber by moving the source relative to the absorber. Due to the velocity of the source a Doppler shift in the energy of the gamma-quantum can match the absorber energy and nuclear resonance will occur.