A SURFACE SPECTROSCOPIC STUDY OF ULTRA-THIN NI FILMS ON MO(110)

The morphology, structures and CO chemisorptive properties of ultra-thin Ni films on a Mo(110) surface have been studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed desorption (TPD), and infrared reflection absorption spectroscopy (IRAS). It is found that Ni grows layer-by-layer on Mo(110) at a sample temperature of 115 K; however, upon annealing to > 600 K, Ni multilayers form three-dimensional clusters. The Ni TPD spectra show two peaks corresponding to desorption from Ni 3D clusters and a 2D uniform monolayer, respectively. Two LEED structures, (8 X 2) and (7 X 2), are observed for Ni/No(110) and are attributed to Ni overlayers with distorted Ni(111) lattices. The (8 X 2) phase is found to transform to the (7 X 2) phase at a Ni coverage of approximately 1 monolayer. This phase transition is also observable in the CO IR spectra. CO adsorbed on the (8 X 2) and (7 X 2) Ni structures shows characteristic stretching frequencies at 2087 and 2065 cm-1, respectively. In addition, the formation of the Ni(7 X 2) structure from disordered Ni is also reflected in the CO IR spectra. The enthalpy difference between the (7 X 2) and the disordered phases is determined to be 3.5 kcal/mol with the (7 x 2) phase being more stable. Upon the deposition of Ni onto CO/Mo(110) at 80 K, CO is found to spill-over from the Mo substrate onto the Ni overlayer resulting in a CO/Ni/Mo(110) structure. Finally, IR spectra of CO on Ni/Mo(110) surfaces as a function of CO coverage and annealing temperature are presented and discussed in terms of surface structure and CO mobility.