Supersensitive electronic transition in impurity Nd–Nd nanoclusters in CaF$_2$ crystal

A comparison of the theoretical calculations with the results of experiments on high-resolution laser spectroscopy made it possible to reveal the fine structure that arises in the supersensitive ${}^4I_{9/2}\leftrightarrow{}^4G_{5/2}$ transition as a result of a strong coherent interaction in the Nd$^{3+}$ ion pair in the CaF$_2$ crystal and construct the level splitting diagram for the ground and excited states of impurity Nd–Nd nanoclusters.