Friction of molecules at metallic surfaces: experimental approach using synchrotron infrared spectroscopy

Adsorbate dynamics has received increasing interest over the last years, as it has been realized that the electron dynamics of the substrate can be profoundly affected by adsorbates due to couplings between vibrational and electronic states. Persson has developed a model where the scattering of the conduction electrons by the adsorbate results in a friction force between the adsorbate and the metallic substrate. The model predicts that the dipole-forbidden low-frequency adsorbate modes exhibit anti-absorption peaks, and that the broadband infrared (IR) reflectance change displays a characteristic frequency dependence in the anomalous skin-effect region, with an asymptotic limit in the mid-IR. The friction coefficient, which can be extracted from the asymptotic change of the reflectance, equals the inverse of the electronic contribution to the lifetime of the frustrated parallel translation. This model has been verified by recording simultaneously the direct-current (DC) resistance change, the broadband IR reflectance change and the IR features for various adsorbates on Cu(111) thin films epitaxially grown on TiO2(110) substrates. The friction coefficients obtained from the DC resistance change and from the IR reflectance change show good agreement. Coadsorption experiments reveal the marked dependence of the friction coefficient on the adsorbate-induced density of states at the Fermi level (E-F). Using the Newns-Anderson model for chemisorption, the IR reflectance change of C-60 adsorbed on the following noble metal surfaces: Ag(111), Au(110) and Cu(100), indicates a high density of induced states at E-F. (C) 1999 Elsevier Science B.V, All rights reserved.