Calorimetric observation of single He2* excimers in a 100 mK He bath

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