Intermolecular mechanism for multiple maxima in molecular dynamic susceptibility

A simple two-level system is introduced to demonstrate the existence of the intermolecular mechanism of the appearance/disappearance of multiple maxima in molecular dynamic susceptibility at low temperature. With a minimum of two relaxation processes, quantum tunneling induced by the nuclear spin bath, and the direct process due to the spin-phonon interaction, we prove that the existence of a sufficiently wide dipolar field distribution in the experimental sample is the main cause of this phenomenon in zero or weak applied dc field. The correlations between the phenomenon and the applied dc field, temperature, or magnetic dilution of the sample are also investigated. Application to several experimental systems has shown a good agreement between the proposed theory and experiments. Cases with a more complex multiplet spectrum, more relaxation processes, and the possibility of more maxima in the molecular dynamic susceptibility are discussed.