Radiative decay of the metastable ${\mathrm{He}}_2\left(a{}^3{\Sigma }_u^{+}\right)$ molecule in liquid helium

Long-lived phosphorescence has been observed emanating from superfluid helium following excitation by ionizing radiation. This extreme ultraviolet light is predominantly from the radiative decay of metastable ${\mathrm{He}}_2\left(a{}^3{\Sigma }_u^{+}\right)$ molecules and is detected through frequency down-conversion and photon counting techniques. This allows measurement of the phosphorescence lifetime in a low-molecular-density regime that is not dominated by two-body reactions. A phosphorescence lifetime of $13\pm 2\hspace{0.5em}\mathrm{s}$ is observed.