Temperature dependence of ortho-Positronium Annihilation in Room Temperature Ionic Liquids.

Temperature dependence of triplet positronium (ortho-Ps) lifetime in N,N,N-Trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) was obtained with positron energies of 2 keV and 12 keV using the vertical slow positron beamline installed at AIST. Positrons could be injected from the liquid surface of TMPA-TFSI and then the surface structure could be detected even at 100 °C. The ortho-Ps lifetime became shorter at higher temperature in near surface and bulk. It indicated that the Ps bubble structure in room temperature ionic liquids was different from that in usual molecular liquids. Ps bubble size obtained from ortho-Ps lifetime can give information of the size of soft region consisting of van der Waals force surrounded by the stiff structure consisting of the Coulomb force. [ABSTRACT FROM AUTHOR]