Asymmetric line shapes and the electron-hole pair mechanism for adsorbed molecules on surfaces

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Abstract
We derive the expression for the line shape of an isolated vibrational mode of the molecules adsorbed on a metallic surface, considering in addition the dipolar interaction between molecules. We find that the presence of the electron-hole damping mechanism necessarily produces an asymmetric line shape just as in the case of an isolated adsorbed molecule. The observed asymmetric line shape of the stretch vibrational mode of a CO monolayer of identical molecules as well as isotopic mixtures of molecules on the Cu(100) surface is discussed in this context, and the contribution of the electron-hole pair mechanism to the observed line shapes of an incomplete monolayer is revealed.