Co as Adsorbate and Reaction Product in the Systems Fe(110)/Cs+Co and Fe(110)/Cs+Co₂

The effect of preadsorbed Cs on the electronic structure and thermal stability of CO adsorbed at 80 K on Fe(110) has been studied using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). With increasing Cs coverage the O1s and C1s core levels of CO are shifted to lower binding energies (-0.8 eV). In the presence of one-monolayer Cs the thermal stability of CO is enhanced, leading to a decomposition temperature of about 510 K, a value roughly 100 K higher than on the clean Fe(110) surface (~ 420 K). This is explained in terms of an increased back-donation leading to an electrostatic interaction between CO and Cs.

These results are compared to the properties of CO which is formed when CO₂ is adsorbed on the Cs-precovered Fe(110) surface, CO formed from adsorbing CO₂ shows neither core level binding energy shift nor enhanced thermal stability. It is concluded that CO formed from adsorbing CO₂ is not affected by the preadsorbed Cs, i.e. there is no or at least significantly less increased backbonding and therefore no electrostatic interaction between Cs and CO in that system. It seems to be in a state comparable to the state of CO in the Fe(110)/CO adsorption system.