The Temperature Dependence of the Two Positronium Bubble States in Liquid SF6

Positron lifetime measurements have been performed on liquid SF6 in the temperature range from −45°C to 71°C (Tc = 45.65°C). The positron lifetime spectra were resolved into four lifetime components. In the order of increasing lifetimes the four lifetime components are associated with the decay of para-positronium (p-Ps), free positrons, ortho-positronium (o-Ps) from a small bubble state, and o-Ps from a large bubble state. The lifetime of o-Ps annihilating from the large bubble state τ4 increases from 5.7 ns at −45°C to 19.5 ns at 53°C. The lifetime of o-Ps annihilating from the small bubble state τ3 was found to be 2–2.5 ns in the main part of the temperature range studied. Apparently, this is the first observation of two different o-Ps states in a liquid. The intensity I4 (I3) increases (decreases) from 16.9% (16%) at −45°C to 47.2% (6.4%) at the critical point while above I3 and I4 are essentially temperature independent. The large Ps bubble state seems to be similar to the Ps bubble state found in most liquids.