  Theoretical gamma-ray spectra of molecule hexane have been calculated and
compared with the experimental results in both gas (Surko et al, 1997) and
liquid (Kerr et al, 1965) phases. The present study reveals that in gas phase
not all valence electrons of hexane exhibit the same probability to annihilate
a positron. Only the positrophilic electrons in the valence space dominate the
gamma-ray spectra, which are in good agreement with the gas phase measurement.
When hexane is confined in liquid phase, however, the intermolecular
interactions ultimately eliminate the free molecular orientation and
selectivity for the positrophilic electrons in the gas phase. As a result, the
gamma-ray spectra of hexane become an averaged contribution from all valence
electrons, which is again in agreement with liquid phase measurement. The roles
of the positrophilic electrons in annihilation process for gas and liquid
phases of hexane have been recognized for the first time in the present study.
