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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.

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