Nuclear magnetic resonance fluorescence

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Nuclear magnetic resonance fluorescence , or nuclear magnetic resonance scattering, is the absorption of a photon by an atomic nucleus followed by the spontaneous emission of a photon. A photon can only be absorbed by the nucleus if its energy corresponds to the difference between the energies of the ground state and an excited state . Therefore one speaks of resonant scattering. When the atomic nucleus returns directly to the ground state, the emitted photon has the same energy as the absorbed one.

In the Mössbauer spectroscopy , the resonant absorption of photons is used for measuring the splitting of nuclear levels. However, while the photons for excitation of the nuclei in Mössbauer spectroscopy come from a radioactive source, monochromatized synchrotron radiation is used in nuclear resonance scattering , which has some advantages (see below).

While an absorption spectrum is recorded with Mössbauer spectroscopy , with nuclear resonance scattering the number of scattered photons is measured as a function of time. So you measure in the period and not in the frequency domain.

The possible scattering processes can be divided into coherent and incoherent scattering.

Coherent nuclear magnetic resonance scattering

If the nucleus is in exactly the same quantum mechanical state after the photon has been scattered, if there are several nuclei it cannot be decided which nucleus has scattered the photon. Therefore one speaks of coherent scattering.

Coherent scattering can be used to measure the splitting of the core levels: Energy levels that are close to one another cause measurable quantum beats when the photons are coherent . Measurements are usually made in the forward direction of the synchrotron beam used, which is why one speaks of nuclear forward scattering .

Incoherent nuclear magnetic resonance scattering

If the final state after scattering differs from the initial state before scattering, it can be determined which nucleus has scattered the photon and the coherence is lost. Usually this happens when the nucleus interacts with other particles during the scattering (for example phonons ), so that incoherent nuclear resonance scattering is usually also inelastic.

However, incoherent elastic nuclear resonance scattering is also possible, namely when the basic state of the nucleus is degenerate . For example, the nucleus can perform a spin-flip during scattering , the energy does not change, but the final state differs from the initial state.

Incoherent nuclear magnetic resonance scattering can be used to measure the phonon spectrum because of the interaction with phonons. The method selects the phonon spectrum of an isotope in the sample, since with a suitable choice of the photon energy only the nuclei of this one isotope can be excited. By means of incoherent nuclear magnetic resonance scattering, the Lamb-Mössbauer and Debye-Waller factors can also be determined very precisely.

Advantages of nuclear magnetic resonance scattering compared to Mössbauer spectroscopy

  • With nuclear resonance scattering, no radioactive parent nuclide is required, which decays into the excited state of the nucleus to be examined in order to obtain photons of suitable energy. Therefore, many more different isotopes can be examined than with Mössbauer spectroscopy .
  • The energy of monochromatized synchrotron radiation can be scanned in a much larger area around the nuclear resonance than is possible with Mössbauer spectroscopy.
  • Since synchrotron radiation is polarized , direction-dependent measurements can be carried out. Synchrotron radiation is also mostly pulsed, so that quantum beats can be observed.

literature

  • R. Röhlsberger: Nuclear Condensed Matter Physics with Synchrotron Radiation. Springer-Verlag, Berlin / Heidelberg 2004, ISBN 3-540-23244-3 .

Web links

swell

  1. ↑ TUNL.duke.edu , What is nuclear resonance fluorescence? (English).