Long-lived phosphorescence has been observed emanating from superfluid heli
um following excitation by ionizing radiation. This extreme ultraviolet lig
ht is predominantly from the radiative decay of metastable He-2(a (3)Sigma(
u)(+)) molecules and is detected through frequency down-conversion and phot
on counting techniques. This allows measurement of the phosphorescence life
time in a low-molecular-density regime that is not dominated by two-body re
actions. A phosphorescence lifetime of 13+/-2 s is observed. [S1050-2947(99
)01701-1].