Vibrational mode-softening of alkanes on clean and modified Cu and Mo surfaces: Absence of a simple correlation with thermal desorption temperatures

Although the strength of the C-H...M interactions between alkanes and metal surfaces has often been correlated with the vibrational frequencies of so-called "softened" nu(C-H) modes, this correlation is still considered to be controversial in many cases. In this letter, we report vibrational and thermal desorption results to demonstrate that there is no simple correlation between the softened nu(C-H) frequencies and the thermal desorption temperatures of hydrocarbon molecules on Cu and Mo surfaces. In order to determine whether general trends exist, we have performed systematic studies in which we varied the nature of both alkane molecules and metal substrates. We have compared the degree of C-H mode softening for different types of hydrocarbons, including linear, branched, and cyclic alkanes. We have also studied the effect of varying the substrate symmetry by comparing the adsorption of cyclohexane on Cu(100), Cu(110), and Cu(111). Furthermore, we have investigated the effect of chemical modification on the extent of mode softening by comparing the adsorption of normal cyclohexane (C6H12) on Cu(111) and Cu3Pt(111), and of deuterated cyclohexane (C6D12) on Mo(110) and carbide-modified Mo(110). (C) 1998 Elsevier Science B.V.