The pseudoquadrupole interaction

A hyperfine effect has been described which can generate a temperature-dependent shift of the centroid of the resonance lines only in rare circumstances. This is the pseudoquadrupole hyperfine interaction [64]. 

TmCla-bHaO and Tm2(S04)3.8H20 both give decays and have electronic levels separated by 1 K. Decreasing the temperature from 4-2 K 

Although not widely applicable, the observation of the pseudoquadrupole effect can be used to measure separations between electronic states of less than 1 cm.  

Solid State Physics, Supp. No. 1, Academic Press, 1958. 

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