SOLVENT AND TEMPERATURE EFFECTS ON POSITRONIUM ANNIHILATION PARAMETERS AND ON ITS QUENCHING REACTIONS WITH THE FREE-RADICAL HTEMPO

Positron annihilation lifetime spectroscopy (LS) and the Doppler broadening of the annihilation radiation lineshape (DBARL) technique have been used in conjunction to study solvent and temperature effects on both the positronium (Ps) quenching reactions with the free-radical HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) and the annihilation parameters. A change in the nature of the quenching reaction is observed from spin conversion in benzene (Bz), N,N-dimethylacetamide (NDMA), and N-methylformamide (NMFA) to Ps oxidation in metacresol (m-C); it is attributed to a modification of the electron density distribution in the quenching molecule due to hydrogen bonding with the m-C molecules. The data also indicate that the full-width at half-maximum of the momentum distribution associated to para-positronium (p-Ps) resulting from the spin conversion reaction is smaller by a factor of 2 than that for p-Ps directly formed in the positron spur. The ortho-positronium (o-Ps) pickoff lifetime and momentum distribution are shown to give access to physicochemical and structural properties of the solvents.