  We present measurements and theoretical modeling demonstrating the capability
of Doppler Broadened annihilation gamma Spectroscopy (DBS) to provide
element-specific information from the topmost atomic layer of surfaces that are
either clean or covered with adsorbates or thin films. Our measurements show
that the energy spectra of Doppler-shifted annihilation gamma photons emitted
following the annihilation of positrons from the topmost atomic layers of clean
gold (Au) and copper (Cu) differ significantly. With the aid of the positron
annihilation-induced Auger electron spectroscopy (PAES) performed
simultaneously with DBS, we show that measurable differences between the
Doppler broadened gamma spectra from Au and Cu surfaces in the high energy
region of the gamma spectra can be used for the quantification of surface
chemical composition. Modeling the measured Doppler spectra from clean Au and
Cu surfaces using gamma spectra obtained from ab initio calculations after
considering the detector energy resolution and surface positronium formation
pointed to an increase in the relative contribution of gamma from positron
annihilation with valence shell electrons. The fit result also suggests that
the surface-trapped positrons predominantly annihilated with the delocalized
valence shell (s and p) electrons that extended into the vacuum as compared to
the highly localized d electrons. Simultaneous DBS and PAES measurements from
adsorbate (sulfur, oxygen, carbon) or thin film (selenium (Se), graphene)
covered Cu surface showed that it is possible to distinguish and quantify the
surface adsorbate and thin-film composition just based on DBS. DBS of elemental
surfaces presents a promising avenue for developing a characterization tool
that can be used to probe external and internal surfaces that are inaccessible
by conventional surface science techniques.
