Hyperfine splitting definition

We have already mentioned that the choice of an appropriate independent-particle model is a problem in itself, espedally for open-shell systems. One of the effects which are correlation only in the frame of a particular definition is spin polarization. This remark is connected with the question of the role of correlation in spin densities and hyperfine splitting constants in free radicals. 

Fig. 4. Structure of ethyl radical and definition of the dihedral angle 0 for hyperfine splitting due to...

While the Overhauser shift is due to delocalised electrons, the ENDOR experiments require more localised electrons. Both groups were able to observe ENDOR on the effective mass donor resonance but not on the deep donor signal. An illustrative ENDOR spectrum is shown in FIGURE 4, in which the 69 71Ga ENDOR lines are observed. The 69Ga line is clearly split by quadrupole interactions just as in the Oveihauser shift measurements, while the quadrupolar interaction is much less resolved in the 71 Ga line, due to its smaller quadrupole moment. In the three samples investigated by the two groups, the linewidths were all too broad to resolve any hyperfine interaction and so no definitive identification of the residual donor was possible. 

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