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

The morphology, structures and CO chemisorptive properties of ultra-th in Ni films on a Mo(110) surface have been studied using Auger electro n spectroscopy (AES), low energy electron diffraction (LEED), temperat ure programmed desorption (TPD), and infrared reflection absorption sp ectroscopy (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 sh ow 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 w ith distorted Ni(111) lattices. The (8 X 2) phase is found to transfor m 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 adso rbed on the (8 X 2) and (7 X 2) Ni structures shows characteristic str etching frequencies at 2087 and 2065 cm-1, respectively. In addition, the formation of the Ni(7 X 2) structure from disordered Ni is also re flected 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 th e (7 x 2) phase being more stable. Upon the deposition of Ni onto CO/M o(110) at 80 K, CO is found to spill-over from the Mo substrate onto t he Ni overlayer resulting in a CO/Ni/Mo(110) structure. Finally, IR sp ectra of CO on Ni/Mo(110) surfaces as a function of CO coverage and an nealing temperature are presented and discussed in terms of surface st ructure and CO mobility.