Evidence for a triplet state of the self-trapped exciton states in liquid argon, krypton and xenon

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Abstract
The lifetimes of luminescence from liquid argon, krypton and xenon excited by energetic electrons have been studied with such a high electric field that all of the observed decay characteristics have to be attributed to the self-trapped exciton luminescence. In all cases the decay shows two exponential components. The lifetimes of the fast components are in the range from 2 to 5 ns, and major second components have lifetimes of 860+or-30 ns for argon, 80+or-3 ns for krypton and 27+or-1 ns for xenon. These lifetimes are interpreted in terms of two self-trapped exciton states, one predominantly singlet in character, the other triplet; this is the first strong evidence for the triplet assignment. The variation in triplet-state lifetimes is attributed to the different spin-orbit couplings.