The gamma-ray spectra of halocarbons in positron–electron annihilation process

•The inner valence electrons in molecules dominate the Doppler shift.•The outermost atomic s electrons in molecules show dominance in the gamma-ray spectra.•The positron can penetrate deeper inside molecules in positrophilic sites to annihilate with inner valence electrons.

The gamma-ray spectra of the positron annihilation process in methane CH4 and its fully substituent halocarbons CF4, CCl4, and CBr4 have been studied. The theoretical predictions of the inner valence electrons agree well with the experimental measurements for all these molecules. That the outermost s electrons in carbon or halogen atoms dominate the gamma-ray spectra has been confirmed for the first time. The positrophilic site has also been found in these molecules and understanding of annihilation processes in molecules has been enhanced.