A systematic study was made of negative-ion formation by sputtering from a Mo(100) surface with and without adsorbate layers. Correlation with the work-function change when the surface electronic state was modified by a Cs overlayer reveals a possible tunneling mechanism. Tunnel barrier widths and heights were estimated for ${\mathrm{Mo}}^{-}$, ${\mathrm{O}}^{-}$, ${\mathrm{H}}^{-}$, and ${\mathrm{D}}^{-}$ from the experimental data.

• Received 13 December 1977

DOI:https://doi.org/10.1103/PhysRevLett.40.574