Calorimetric Observation of Single He₂^* Excimers in a 100-mK He Bath

We report the first calorimetric detection of individual He₂^* He 2 ∗ excimers within a bath of superfluid ⁴He 4 He . The detector used in this work is a single superconducting titanium transition edge sensor (TES) with an energy resolution of ~ 1~eV ∼ 1 eV , immersed directly in the helium bath. He₂^* He 2 ∗ 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 ≈ 15~eV ≈ 15 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 ≈ 15~eV ≈ 15 eV , released upon decay, into the surface. We also briefly discuss the detector fabrication and characterization.