Magnetic interaction: NMR measures

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  • Published: Mar 15, 2018
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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Electronic interaction

An international team has proposed a solution to the

How do electrons interact with the atomic nucleus? It seems that we thought we knew the answer, but physicists at TU Darmstadt, Germany cast doubt on our current understanding last year. The same team has now proposed a solution to the "hyperfine puzzle" they raised in their original paper. They report new measurements of the magnetic properties of bismuth atomic nuclei in the journal Physical Review Letters.

The visible spectrum of an atom results from the interplay of the light and the electrons in the atom. Ultra-precise measurements can reveal the hyperfine structure of the atom but the team in Germany spotted a discrepancy when measuring the hyperfine structure of highly charged ions with few remaining electrons. Theory and the experimental splitting did not match. This puzzle immediately raised the question as to whether the interplay between a limited number of electrons in a highly charged ion and atom's nucleus were being affected by strong magnetic fields in ways we did not yet fully understand. The obvious experiment was to determine the strength of the magnetic field within the atomic nucleus, so that the theoretical predictions could be reconciled with the experimental data.

Wilfried Nörtershäuser and Michael Vogel and their teams collaborated to measure the strength of the magnetic moment of the bismuth nucleus using nuclear magnetic resonance (NMR) spectroscopy. To carry out the experiment, the researchers introduced an aqueous solution enriched with bismuth ions to a superconducting magnet and irradiated it with radio waves and observed polarity reversal in the bismuth ions. Of course, they also had to take into account the ions' environment, i.e., the atoms to which it is bound as well as the fluid in which it is dissolved, all of which can change the external magnetic field in the vicinity of the atomic nucleus, which, in turn, affects the precise measurement of the magnetic moment.

Disruptive effects

Such disruptive effects can be subtracted from the calculation using quantum-theoretical calculations. These were carried out by colleagues at the University of St. Petersburg, Russia, and at the Helmholtz Institute Jena. The researchers report that it quickly became obvious that the effect was much larger than previously expected when using bismuth-nitrate solutions, which means that measurements taken using bismuth-nitrate solutions are moot. To sidestep this problem, the tea used an organometallic complex , which releases hexafluoridobismuthate(V) ions in organic solution. It was thus possible to measure much narrower resonance curves and to make more precise statements about the magnetic moment of the nucleus. From the quantum-theoretical perspective, much more precise calculations could be carried out than with the bismuth nitrate approach.

The team has now used the newly calculated value for the magnetic moment of the stable bismuth isotope and made a theoretical prediction of the hyperfine structure splitting within the highly charged ions. The new values agree well with experimental data from laser spectroscopy.

Magnetic fields

"It would be too early to state that this represents the complete solution to the hyperfine puzzle," concedes Nörtershäuser. "Nevertheless, it is a significant part of the solution." He adds that further experiments are now needed to achieve complete clarity about the interplay between the atomic nucleus and the shell and, therefore, to verify the theoretical predictions of the nature of quantum mechanics in very strong fields.

Related Links

Phys Rev Lett 2018, 120, online: "New Nuclear Magnetic Moment of Bi209: Resolving the Bismuth Hyperfine Puzzle"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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