Slow (<5 eV) positrons ($e^{+}$) are observed emanating from clean (submonolayer contaminated) single-crystal metal surfaces which are being bombarded by 0.1-3-keV $e^{+}$. A portion of the emitted slow $e^{+}$ from Al(100) has a narrow range of energies and a temperature-dependent intensity which suggests that these $e^{+}$ are thermalized in the bulk and have a negative work function ${\varphi }_{+}=-0.2\mathrm{}\pm 0.1$ eV. A higher-energy portion of the slow-$e^{+}$ spectrum has a temperature-independent intensity attributable to nonthermalized $e^{+}$.

• Received 4 August 1978

DOI:https://doi.org/10.1103/PhysRevLett.41.1076