Abstract
Doppler-broadened spectra and lifetime spectra from positron annihilation have been measured on zone-refined hexamethylbenzene (HMB) and hexachlorobenzene (HCB) single crystals, in which planar aromatic molecules lie nearly in a plane. The former spectra are found to be dependent on the direction of the crystals and widest in the direction perpendicular to the molecular planes. This phenomenon is explained in terms of an anisotropic momentum-distribution of π -electrons. The latter spectra are found to consist of four components, revealing the formation of positronium (Ps) in these highly-purified organic crystals at room temperature. A Doppler-broadened spectrum of HMB sharpened by a magnetic field indicates that delocalized p-Ps moves much more freely along the molecular planes.
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