Mössbauer Spectroscopy

Alone, to give and lose,
Far partner, never seen.
Others, to join and help,
Give all, take everything.

Nuclei in atoms undergo a variety of energy level transitions, often associated with the emission or absorption of a gamma ray. In a free atom the nucleus recoils, due to conservation of momentum, resulting in the emitted gamma ray being of lower energy than the nuclear transition energy. The same is observed in absorption where the absorbing nucleus recoils, meaning the energy of the resonantly absorbed photon has to be greater than that of the transition.

Thus, in these circumstances, resonant emission or absorption doesn't occur.^2.1 In 1957 Rudolph Mössbauer discovered the phenomenon of Recoil-Free Nuclear Resonance Fluorescence [4]; a phenomenon later to become commonly known as the ``Mössbauer effect''.