Origin of two weak spectral features in the full-range VUV comb scan of 229Th:CaF2

Determine the physical origin of the two weak spectroscopic lines observed during the full-range vacuum‑ultraviolet frequency‑comb scan of the 229Th nuclear clock transition in a CaF2 host crystal at 150 K, by ascertaining whether these features arise from 229Th nuclei incorporated in alternative charge‑compensation configurations or other distinct electronic environments within the CaF2 lattice, and by characterizing their associated frequency shifts and splittings relative to the nuclear quadrupole structure.

Background

In the reported VUV frequency-comb spectroscopy of 229Th:CaF2, six spectroscopic features were observed over a full-range scan. Four dominant peaks were assigned to electric quadrupole splittings of the I = 5/2 to I = 3/2 nuclear transition and used for absolute frequency determination, while a fifth, weaker line was later confirmed at the predicted frequency using a sum rule.

Two additional weak lines, highlighted in the full-range scan (Fig. 2), do not fit into the assigned quadrupole structure. The authors suggest these could originate from 229Th atoms in different charge-compensation configurations or other electronic environments in the CaF2 host, but their exact origin is not yet established. Identifying the mechanism behind these features is important for materials engineering and systematic control in solid-state nuclear clock implementations.