Estimating variations in surface chemical composition through the analysis of the high momentum region of the Doppler broadened annihilation gamma spectra

Doppler broadened annihilation gamma spectrum (DBGS) measured after depositing low energy positrons on bilayer graphene on polycrystalline copper is compared to the spectrum obtained from the same sample surface but after the partial removal of the bilayer graphene film. DBGS from the partially sputtered surface shows a measurable change from the spectrum collected from the non-sputtered surface. We show that the observed variations in the annihilation gamma line shape can be used to quantify the surface chemical composition variation brought about by partial sputtering. The composition variation obtained from Doppler broadening spectroscopy was comparable to the changes observed in the intensity of the copper Auger peaks seen in the positron annihilation-induced Auger electron spectra (PAES). The results show the sensitivity and surface selectivity of the Doppler broadened annihilation gamma spectroscopy in obtaining the surface chemical composition. Our results provide impetus to the development of a new technique based on Doppler broadened annihilation gamma spectroscopy for probing the hidden or inaccessible surfaces of technologically important porous materials.