Temperature dependence of the NMR shift and line width in Van Vleck paramagnets

The temperature dependences of the NMR shift and line width in thulium ethylsulphate located in a magnetic field parallel to the crystal axis are studied theoretically and experimentally. The model of random variation of the resonance frequency due to relaxation transitions between three electron states may be applied to the system. The theory is in good agreement with the experiments for small correlation times tau. At low temperatures (corresponding to those in the experiments) the role of tau-1 is played by the probability per unit time of transitions from excited electron states to the ground state. The smallness of the correlation time justifies the phenomenological approach in accordance with which the resonance frequency shift is proportional to the static paramagnetic susceptibility of the crystal