CO interaction with co-adsorbed C2H4 on Cu(111) as revealed by friction with the conduction electrons

Simultaneous measurements of the DC resistance and IR reflectivity changes of thin epitaxially grown copper (111) films have been performed during co-adsorption of CO with C2H4. The linear dependence of these two quantities, which has previously been observed for CO on Cu(111) films, also holds for this coadsorbed system. Importantly, we have shown that the linearity factor is independent of the adsorbate type, as predicted by the scattering model of the conduction electrons. Co-adsorption of CO with C2H4 results in markedly more pronounced changes in IR reflectance and in resistance; these changes are higher than the sum of the individual changes of each species. The results are interpreted as a consequence of covalent through-metal bonding of the adsorbates via the lowest unoccupied molecular orbitals.