Metallic quantum well states and chemisorption quantum size effects in metal thin film systems

We have studied the electronic properties of Cu/Fe and Cu/Ni metal thin film systems using inverse photoemission spectroscopy (IPS) and photoemission spectroscopy (PES). These systems exhibit metallic quantum well (MQW) states which are electronic states localized to the Cu overlayer. We have measured the energy dispersion of these MQW states as a function of Cu overlayer thickness and also momentum parallel to the surface of the overlayers. Additionally, we have studied the effect of the underlying Fe and Ni layer on the MQW dispersions, and we modeled our experimental studies with a phase accumulation model (PAM). At the center of the Brillouin zone, we observe that both the occupied and unoccupied MQW states in the Cu/Fe system increase in energy as a function of Cu thickness, and these states pass through the Fermi energy ( EF) of the system with a period of 5--6 Cu monolayers (MLs). These results are consistent with the magnetic oscillation periods observed in related metal thin film systems and strengthen the connection between MQW states and the magnetic properties of these systems. The PAM calculations are in good qualitative agreement with the experimental dispersions as a function of parallel momentum, but there are quantitative differences that are most likely the result of simplifying assumptions used in the model. Similar to the MQW states of Cu/Fe, the occupied MQW states of the Cu/Ni system also increase in energy as a function of Cu thickness and pass through EF with a period of 5--6 MLs. The unoccupied MQW states, however, exhibit markedly different behavior; our IPS results show unoccupied features that instead decrease in energy as a function increasing Cu thickness.; We have also studied the influence that MQW states have on the adsorption of carbon monoxide (CO) on the Cu/Fe and Cu/Ni systems. By using temperature programmed desorption (TPD), we observed that the temperature at which the rate of CO desorption is a maximum, Tdes, oscillates as a function of Cu overlayer thickness in the Cu/Fe system. The change in Tdes is significant, and the value of Tdes increases from ∼172 K at a Cu thickness of 2.5 MLs to ∼180 K at 5 Cu MLs. Furthermore, the value of Tdes exhibits maxima at Cu thicknesses of 5 MLs and ∼10--11 MLs, two thicknesses at which MQW states were observed to cross EF as measured by PES and IPS. This observation indicates that MQW states not only play a significant role in CO adsorption, but that the chemical properties of this system can be modified in a systematic way simply by changing the thickness of the film. (Abstract shortened by UMI.)...