Direct observation of a highly forbidden optical transition in Sm:SrF$_2$

The $4f^6$ $^{7}F_0$ $\to 4f^6$ $^{5}D_0$ intra-configuration transition in Sm:SrF$_2$ is forbidden for Sm$^{2+}$ ions in the octahedrally symmetric substitution sites in SrF$_2$. We report the direct observation of this transition using laser-induced fluorescence at cryogenic temperatures, and measurements of the excited state lifetime and the excitation cross section. To the best of our knowledge, this optical transition has the longest lived excited state ever observed in a solid.