Radiative decay of the metastable He2(a(3)Sigma(+)(u)) molecule in liquid helium

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].