Paper SS1-TuA11
Modifying the Adsorption of Molecules at Metal Surfaces by Quantum Confinement of Electrons

Tuesday, October 16, 2007, 5:00 pm, Room 608

Nanoscale metal thin films can exhibit quantum size effects (QSE) whereby their electronic, structural, magnetic and chemical properties may differ greatly from those of the bulk. Quantum confinement of electrons forms so-called metallic quantum well (MQW) states which give rise to many of these interesting phenomena. In previous studies we found that MQW states modify the strength of CO bonding to Cu MQW overlayers on the pseudomorphic fcc-Co/Cu(100) and fcc-Fe/Cu(100) systems.¹ In these systems, the CO-metal bond strength oscillates, in correlation with MQW states, as a function of Cu overlayer thickness. Here we extend these studies with IR studies of CO adsorbed on Cu MQWs and report on the adsorption properties of the pseudomorphic transition metal layers themselves. These systems have partially filled d-bands may be grown in a metastable structure. We have examined the unoccupied electronic structure and CO bonding strength on the n-ML fccNi/Cu(100) and n-ML fccCo/Cu(100) systems using inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). In the case of Co, the as-grown films exhibit well-defined unoccupied MQW states, but they do not cross the Fermi level. This allows us to investigate adsorbate-MQW interactions that involve MQW states away from the Fermi level. Upon CO adsorption a well-defined structure centered about 3.8 eV above the Fermi level appears and is assigned to the unoccupied CO 2π orbital. CO adsorbs molecularly at room temperature and in TPD measurements we find a desorption temperature of approximately 375 K, which is about 30 K lower than what is observed for CO adsorbed on the hcp Co surfaces. When Co films are dosed at low temperatures (~ 100K), we find a second CO desorption peak around 230 K, once again similar to what is seen for hcp Co, but as a markedly lower temperature. These peak desorption temperatures change as a function of film thickness and are correlated with two different C-O stretch vibrational frequencies observed in the IR spectra. We have observed similar desorption peak temperature shifts when CO desorption from the Ni/Cu(100) system as a function of thickness and as compared to single crystal Ni(100). The role of quantum confinement and surface strain in producing the observed effects will be discussed.

¹ A. G. Danese, F. G. Curti, and R. A. Bartynski, Phys. Rev. B 70, 165420 (2004).