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Originally Posted by papageno
This is the typical situation for Mössbauer effect.
Take a crystal lattice containing radioactive (gamma emitting) Iron isotopes.
The excited Iron nuclei emit gamma photons. If the energy of that photon is lower than the energy necessary to excite a phonon (quantum of lattice vibrations), the emission of the gamma photon is effectively recoil-less. The recoil is transferred to the whole crystal (10^23 atoms), rather than to the emitting atom (which would make the atom vibrate in the lattice).
The spectral line corresponding to this recoil-less emission, is very narrow, and makes some nice experiments possible.
For example, it has been used to measure gravitational red-shift on Earth, between the basement and the top of a tower.
In a teaching lab, I used it to measure the hyperfine field in Iron.
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This much I think I understand.
As a test of my understanding... it seems to me that the recoil-less emission should mean that photons can be absorbed by an atom, and then re-emitted with exactly the same frequency to all intents and purposes, with no energy loss to the lattice. The re-emitted photon can continue to pass through the lattice, possibly being absorbed and re-emitted a number of times, all with no loss of energy. But so far I see nothing to constrain it to remain in a straight line.
I've seen references compare this with "resonance fluorescence of the yellow D lines of sodium in sodium vapour". See, for example
Recoilless nuclear resonance absorption of gamma radiation, which is Rudolf Mössbauer's Nobel lecture. In this case, the fluorescence means that the whole gas glows as photons bounce around within the gas. I guess the same thing happens for gamma photons in a crystal lattice at the excitation frequency of the Mössbauer effect.
If a beam of gamma ray photons of exactly the right frequency are directed into the lattice, I would expect to see photons emerging from the lattice at the same frequency and in every possible direction.
Have I got this right? When I first read Lyndon's paper, I got the impression that Mössbauer effect meant that the photons had no scatter angle, but now I think this is just another error on Lyndon's part. Or perhaps it is my error and I have misunderstood Mössbauer's lecture.
Unless I am badly mistaken, Lyndon's effect is completely the opposite of the Mössbauer effect in all the essential respects:
- The Mössbauer effect works only for very very tightly constrained frequencies. The Lyndon effect allegedly works right across the spectrum.
- The Mössbauer effect is distinguished by no recoil and no redshift. The Lyndon effect allegedly involves a redshift to the photon and two recoils to an electron.
- The Mössbauer effect involves something comparable to fluorescence, in which a lattice "glows" with gamma ray photons emerging in all directions. The Lyndon effect allegedly involves no scattering at all, with the photons emerging with direction unchanged.
It is the last point on which I am least certain.
Thanks for any help -- Sylas