AVS 52nd International Symposium
    Surface Science Thursday Sessions
       Session SS1-ThA

Paper SS1-ThA8
Diffusion and Ordering of CO Coadsorbed with H on Pd(111) Studied by Variable-Temperature STM

Thursday, November 3, 2005, 4:20 pm, Room 202

Session: Transport and Structural Stabilization of Surfaces
Presenter: D.F. Ogletree, Lawrence Berkeley National Laboratory
Authors: T. Mitsui, Lawrence Berkeley National Laboratory
M.K. Rose, University of California, Berkeley
E. Fomin, University of California, Berkeley
D.F. Ogletree, Lawrence Berkeley National Laboratory
M. Salmeron, Lawrence Berkeley National Laboratory
Correspondent: Click to Email
CO adsorbs on Pd(111) in fcc hollow sites at low coverage. As coverage increases, ordered @sr@3x@sr@3 domains form with 1/3 of fcc sites occupied by CO. At higher coverage c(4x2) domains form with a density of 1/2, with CO adsorbed in either bridge sites, or in fcc and hcp hollow sites. Additional CO structures are formed at still higher coverage. H also adsorbs on Pd(111) in fcc sites, forming ordered @sr@3x@sr@3 domains with first 1/3 then 2/3 of fcc hollow sites occupied by H. At still higher coverage, H occupies all fcc sites forming 1x1 domains.@footnote 1@ When H was adsorbed on Pd(111) pre-covered with 0.12 to 0.33 ML of CO, the CO was compressed into c(4x2) domains, however higher-density CO domains were not observed even in a constant background pressure of 10@super -7@ Torr H@sub 2@. Co-adsorption of H has little effect on the diffusion of isolated CO molecules until the H coverage approaches 1/3 ML. At this point the CO diffusivity drops more than two orders of magnitude. CO diffusivity rises significantly as the H coverage passes 1/3 ML, then drops again as coverage approaches 2/3 ML. Very stable CO clusters were observed as H coverage approached 1 ML. Isolated CO molecules could be followed by STM up to ~ 110 K, while isolated CO diffusion on clean Pd(111) can be observed by STM as low as 45 K, and becomes to fast to follow by ~ 55 K. In small clusters CO molecules occupy both fcc and hcp sites, and diffusivity is a dramatic function of cluster size. 2 and 4 molecule clusters diffuse significantly faster than 1, 3 or 5 molecule clusters, and cluster diffusivity generally increases with size up to at least 7 CO molecules. The implications of these observations for inter-adsorbate interactions will be discussed. @FootnoteText@ @footnote 1@ M. K. Rose et al, Surface Science 2002, T. Mitsui et al, Surface Science 2003, T. Mitsui et al, Phys. Rev. Lett. 2005.