SERS-active sites on various copper substrates

The wavenumbers of the Raman bands of ethylene adsorbed on Cu in ultra-high vacuum depend on the surface structure of the copper substrate. Thus it is possible to differentiate between ethylene adsorbed at Cu(110) sites, Cu(1 11) sites and surface defect sites of unknown configuration. These latter a re the 'SERS-active sites.' Only when these sites are annealed does the dis tance dependence of SERS follow the expectations from the electromagnetic m odel of SERS. In the presence of the SERS-active sites, also the signal of ethylene at(111) sites shows a 'first layer effect.' Ethylene adsorbed on c old-deposited copper films shows strong infrared absorption at the same wav enumber as the ethylene at SERS-active sites in Raman scattering, irrespect ive of the Raman-infrared exclusion rule for the free centrosymmetric ethyl ene molecule. This raises the possibility that SERS-active sites are also i nfrared-active sites. The SERS-active sites at stepped eu surfaces decorate d by 3 nm of cold-deposited copper anneal below 200 K, whereas they prevail beyond 400 K on Cu island films. Copyright (C) 2001 John Wiley & Sons, Ltd .