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We present measurements and theoretical modeling demonstrating the capabilityof Doppler Broadened annihilation gamma Spectroscopy (DBS) to provideelement-specific information from the topmost atomic layer of surfaces that areeither clean or covered with adsorbates or thin films. Our measurements showthat the energy spectra of Doppler-shifted annihilation gamma photons emittedfollowing the annihilation of positrons from the topmost atomic layers of cleangold (Au) and copper (Cu) differ significantly. With the aid of the positronannihilation-induced Auger electron spectroscopy (PAES) performedsimultaneously with DBS, we show that measurable differences between theDoppler broadened gamma spectra from Au and Cu surfaces in the high energyregion of the gamma spectra can be used for the quantification of surfacechemical composition. Modeling the measured Doppler spectra from clean Au andCu surfaces using gamma spectra obtained from ab initio calculations afterconsidering the detector energy resolution and surface positronium formationpointed to an increase in the relative contribution of gamma from positronannihilation with valence shell electrons. The fit result also suggests thatthe surface-trapped positrons predominantly annihilated with the delocalizedvalence shell (s and p) electrons that extended into the vacuum as compared tothe highly localized d electrons. Simultaneous DBS and PAES measurements fromadsorbate (sulfur, oxygen, carbon) or thin film (selenium (Se), graphene)covered Cu surface showed that it is possible to distinguish and quantify thesurface adsorbate and thin-film composition just based on DBS. DBS of elementalsurfaces presents a promising avenue for developing a characterization toolthat can be used to probe external and internal surfaces that are inaccessibleby conventional surface science techniques.

Measurement and analysis of the Doppler broadened energy spectra of  annihilation gamma radiation originating from clean and adsorbate-covered  surfaces