Calorimetric Observation of Single \(\hbox {He}_2 angle \) Excimers in a 100-mK He Bath

We report the first calorimetric detection of individual \(\hbox {He}_2 angle \) excimers within a bath of superfluid \( pound sterling \hbox {He}\). The detector used in this work is a single superconducting titanium transition edge sensor (TES) with an energy resolution of \({\sim }1~\hbox {eV}\), immersed directly in the helium bath. \(\hbox {He}_2 angle \) excimers are produced in the surrounding bath using an external gamma-ray source. These excimers exist either as short-lived singlet or long-lived triplet states. We demonstrate detection (and discrimination) of both states: In the singlet case the calorimeter records the absorption of a prompt \({\approx }15~\hbox {eV}\) photon, and in the triplet case the calorimeter records a direct interaction of the molecule with the TES surface, which deposits a distinct fraction of the \({\approx }15~\hbox {eV}\), released upon decay, into the surface. We also briefly discuss the detector fabrication and characterization.