Competing routes for charge transfer in co-adsorption of K and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>on graphite

We have investigated a model co-adsorption system using electron energy loss spectroscopy. When the dispersed phase of K graphite is exposed to ${\mathrm{O}}_{2}$ at 25 K, the charge donated by the alkali is depleted, as evidenced by a reduction of the surface plasmon energy from \ensuremath{\sim}320 to \ensuremath{\sim}130 meV. The co-adsorbed ${\mathrm{O}}_{2}$ species is molecular, with a vibrational frequency consistent with the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{g}}$ ground state of ${\mathrm{O}}_{2}^{\mathrm{\ensuremath{-}}}$. The results demonstrate local transfer of charge from alkali to co-adsorbate, consistent with a simple electrostatic model. By contrast, CO perturbs the K-graphite interaction only weakly.