Competing routes for charge transfer in co-adsorption of K andO2on 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 ∼320 to ∼130 meV. The co-adsorbed ${\mathrm{O}}_2$ species is molecular, with a vibrational frequency consistent with the ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$ ground state of ${\mathrm{O}}_2^{\mathrm{-}}$. 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.