| The Mössbauer Effect, also called Recoil-Free Gamma-Ray Resonance
Absorption is a nuclear process permitting the resonance absorption of
gamma rays. This is possible when atoms are fixed in a lattice, and
the energy associated with the recoil of the nucleus due to photon
emissions and absorption is smaller than the energy of the lattice
vibrations. This process was discovered in 1957 by German physicist
Rudolf L. Mössbauer, for which he received the Nobel Prize in 1961.
Suppose we have two identical atoms; one with its nucleus in the
excited state, the other in the ground state. The excited nucleus will
decay to the ground state, while emitting a photon with an energy
around 10-100 keV. If the photon interacts with the nucleus of the
atom that was in the ground state, this will generally not
bring the nucleus in the excited state. This is due to losses in the
form of recoil energy. When the excited nucleus decays and emits a
photon, it will recoil (like a gun does from firing a bullet).
This recoil energy, ERis equal to
E0/2mc2, where m is the mass of the nucleus, and c
is the speed of light. Thus the photon energy Eγ
equals E0-ER. Similarly, if the photon hits the
second nucleus, this will also recoil with an energy ER,
leaving only E0-2ER available for further
excitations. It is clear that in the case of free moving atoms, the
experiment to absorb emitted photons fails due to recoil effects.
If the atoms are fixed in a solid lattice, they cannot recoil as if
they were free (compare this to the recoil of a loosely held rifle
versus one that is firmly held to the shoulder.) In this case the recoil
energy will not simply be taken up by the nucleus, but by vibrations of
the lattice as a whole. These lattice vibrations are quantized by
phonons. If the recoil energy due to emission of a
photon is larger than a phonon energy quantum, the lattice will simply
absorb the recoil energy. However, if the recoil energy is smaller than
the phonon energy quantum, this will lead to an important quantum
mechanical effect; a number of emission or absorption effects will take
place without exchange of recoil energy. This occurrence is called the
Mössbauer Effect.
Only specific elements exhibit the Mössbauer Effect, such as
iron, tin, iridium, ruthenium, antimony, platinum and
gold. The effect can be used in spectroscopy to analyze energy levels
of an atomic nucleus with very high accuracy and thus reveal the
oxidation state and strength of the magnetic field around the
nucleus. This technique is called Mössbauer
Spectroscopy. | 
