Enhanced Infrared Absorption of SERS-Active Lines of Ethylene on Cu

Abstract

Vibrational modes of adsorbates on rough metal films can show surface enhanced Raman scattering (SERS) as well as surface enhanced infrared (IR) absorption (SEIRA). Both effects are a consequence of field enhancement and of a so-called chemical effect. Our experiments show that chemical contributions to SERS and to SEIRA from ethylene on Cu may have the same origin. We investigated ethylene adsorbed on ultrathin Cu films at about 100 K and below with IR transmission spectroscopy. At exposures up to 2 L the IR spectra show lines only at the positions of the very intense SERS E-bands. At higher exposures lines at the frequencies of the infrared active vibrations of ethylene molecules develop. All lines are stronger than one expects from molecular IR activities. Co-adsorption experiments performed with ethylene and CO show that E-lines in SEIRA do not occur after pre-coverage of Cu with CO.