Adsorption of carbon monoxide on copper (100) studied by photoelectron spectroscopy and low energy electron diffraction

Abstract

By combining u.v. and X-ray photoelectron spectroscopy with low energy electron diffraction, Cu(100) has been shown to adsorb carbon monoxide at 295 K. Adsorption, although molecular, has distinctly different spectroscopic characteristics from adsorption at 80 K. The oxygen (1s) binding energy is 1.3 eV lower at 295 K than at 80 K, the carbon 1s binding energy is also lower. The sticking probability is ∼10^–6 at 295 K although close to unity at 80 K, and the energy of desorption is greater for the room temperature state. The LEED pattern, (√2 ×√2)R45° is the same as observed at 80 K (for θ < 0.5) but the maximum coverage is substantially smaller at 295 K (θ∼ 0.3) and the diffraction spots more diffuse. At 80 K the photoelectron spectroscopy and LEED data are similar to those observed previously. Changes in the u.v. induced spectra with increasing CO coverage are shown to reflect the onset of LEED “compression structures” rather than the presence of two adsorbed phases of carbon monoxide. We suggest that the existence of two distinct types of CO bonding on copper (100), (at 295 and 80 K) is a consequence of the molecule acting more as an electron acceptor, (“CO^δ–”) at 295 K and more like an electron donor, (“CO^δ+”) at 80 K. The presence of a chemisorption level 1.5 eV below the Fermi edge, i.e. in the s-band region of the spectrum, is interpreted as being due to reorganisation of metal s and d electrons which is in keeping with the transition metal-like characteristics of the adsorption at 295 K.