Magnetic field effects spin selection rule

Fig. 5. Ground and excited states of (a) high-spin and (b) low-spin ferric hemes, including the effects of spin-orbit coupling (SOC) and a 2-directed magnetic field, B. The selection rules for right (R) and left (L) circularly polarized light are indicated (adapted from Ref. 45).

In this chapter, we review electronic structure in hydrogenlike atoms and develop the pertinent selection rules for spectroscopic transitions. The theory of spin-orbit coupling is introduced, and the electronic structure and spectroscopy of many-electron atoms is greated. These discussions enable us to explain details of the spectra in Fig. 2.2. Finally, we deal with atomic perturbations in static external magnetic fields, which lead to the normal and anomalous Zeeman effects. The latter furnishes a useful tool for the assignment of atomic spectral lines. 

There was an early semiempirical theory of relativistic effects with the main message that the changes in selection rules brought about by spin-orbit interaction have a large effect on chemical shifts. Some insight can also be gained from inspection of the Pauli limit of relativistic theory in the presence of a magnetic field. 

The effects of hyperfine structure. In their original investigation Brossel and Bitter (1952) also performed double-resonance experiments on the odd mercury isotopes Hg and Hg which have nuclear spin I =1/2 and 3/2 respectively. The r.f. magnetic field now induces transitions between the hyperfine structure levels F,Mp> which satisfy the selection rule AF 0, AMp = +1. In low magnetic fields the observed resonances allow the gp factor (equation (15,28)) of a given hyperfine level to be determined and so lead to a determination of the nuclear spin I (Problem 16.5). 

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