High resolution spectroscopy and characterization of matrix-isolated thulium

Narrow linewidths of ground state transitions in thulium defects embedded in argon crystal make it a competitive candidate for solid-state applications of quantum information processing and sensing. High resolution absorption spectroscopy reveals that the ²F_7/2 - ²F_5/2 (1140 nm) transition is split into multiple components with linewidths as small as 200 MHz. We propose that the splitting is due to crystal field effects at multiple trapping sites and hyperfine interactions, and we characterize a family of lines belonging to a single trapping site.