Investigations of fullerenes using positron annihilation spectroscopy

Positron lifetime measurements at low temperatures across the structural transformations in solid C-60 and C-70 are presented. Experiments above room temperature, upto 400 degrees C, indicate a reversible increase in positron lifetime, which is accounted for in terms of thermally activated trapping of positrons from the interstitial regions to the open regions within the cage structure. In addition to studies in solid state, the annihilation characteristics of the ortho positronium (O-Ps) formed in organic solvents has been monitored with increasing concentration of dissolved C-60 and C-70. The observed reduction of the O-Ps lifetime is understood in terms of the formation of O-Ps-Fullerene complex prior to annihilation.