Electronic contributions to infrared spectra of adsorbate molecules on metal surfaces: Ethene on Cu(1 1 1)

Abstract

Already at coverages well below 1 ML copper ad-atoms on Cu(1 1 1) significantly modify infrared reflection-absorption spectra of ethene (C2H4) on Cu(1 1 1). Raman modes of the centrosymmetric molecule appear in the spectra. Their lines do not involve significant shifts of vibration frequencies as they would be expected for strong distortions of the molecule. High-resolution electron-energy-loss spectra of adsorbed ethene for various Cu-adatom precoverage show the same vibration lines. Since the molecules are obviously unchanged, the Raman lines in the infrared spectra must have obtained dynamic dipole moment from transient electron transfer favored by atomic disorder on the metal surface.