Effective electronic magnetic momenta

Frank s research career began at a time when Solid State Physics was a new topic in Physics. He contributed very much to what became an all-embracing topic in both basic physics and in its numerous applications in chemistry as well as physics in areas now frequently described as Condensed Matter . He pointed out how random strains can force a dynamic Jahn-Teller system to reflect distorted static behaviour in certain cases particularly in EPR spectra. This involved considerations of the orbital angular momentum of the magnetic ions present in these systems and he showed how the Ham Effect , as it became known, could explain why the electronic angular momentum could be quenched in many systems. 

The origin of this magnetic moment was not clear in 1922. In its electronic ground state, a silver atom does not possess a spatial angular momentum, and the concept of an intrinsic electronic angular momentum (the electron spin) was yet to be created. In 1925, Goudsmit and Uhlenbeck introduced a fourth (spin) electron degree of freedom—in addition to the three spatial coordinates (x, y, z)—as a model to ease the explanation of the anomalous Zeeman effect.3,4... 

These relations can be used to calculate the nuclear magnetic momentum if / is known, or vice versa. The nuclear field experienced by the nucleus is not exacdy equal to the external field because of the shielding effect of the electron shell, which depends on the electron structure. Although this shielding effect is very small, it can easily be measured by use of NMR spectrometers. 

In atoms or molecules containing several electrons, only unpaired electrons present in singly occupied atomic or molecular orbitals show an effective magnetic momentum. 

As mentioned above, magnetic neutron scattering occurs due to the interaction of the neutron s magnetic moment with unpaired electrons of momentum p and spin s (if we discount the generally significantly smaller nuclear moments [3]). The interaction potential arises from two sources the inherent spin of the electron and the orbital motion of the electron around the nucleus. In some cases, such as the transition metals, the orbital part of the interaction is effectively quenched by the crystal structure. 

The notion of electronic spin was first proposed by Uhlenbeck and Goudsmit in 1925 to account for the sphtting of some of the lines seen in atomic spectra. They and others showed that the electronic spin also accounted for the anomalous effects of magnetic fields (Zeeman effects) on the spectra of many atoms. However, it was necessary to postulate that the magnetic moment associated with electronic spin is not simply the product of the angular momentum and ejlmc, as is true of orbital magnetic moments, but rather twice this value. The extra factor of 2 is called the Lande g factor. When Dirac [33] reformulated quantum mechanics to be consistent... 

The Lande factor describes the effective magnetic momentum of an atom or electron, in which the orbital angular momentum L and the spin angular momentum S are combined to give a total angular momentum J. 

A new type of the isotope effect, viz., magnetic isotope effect, has recently been discovered. The theray of influence of the magnetic field on the rate of chemical reactions is based on the fundamental law of angular momentum conservation. Naturally, this law also concerns the intrinsic angular momentum of electrons and nuclei (spin). Therefore, any changes in the total spin arc... 

But similar calculations for the iron-group ions show marked disagreement with experiment, and many attempts were made to explain the discrepancies. The explanation is simple in many condensed systems the perturbing effect of the atoms or molecules surrounding a magnetic atom destroys the contribution of the orbital momentum to the magnetic moment, which is produced entirely by the spin moments of unpaired electrons.40... 

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