AVS 47th International Symposium
    Surface Science Wednesday Sessions
       Session SS2+VT-WeA

Paper SS2+VT-WeA6
The Effect of Deposition Pressure on Adsorbate Structure and Coverage: Oxygen on W(110)@footnote 1@

Wednesday, October 4, 2000, 3:40 pm, Room 209

Session: Adsorption and Desorption Phenomena I
Presenter: S. Chiang, University of California, Davis
Authors: D.E. Muzzall, University of California, Davis
C.S. Fadley, University of California, Davis and LBNL
S. Chiang, University of California, Davis
Correspondent: Click to Email
In most prior surface science studies, it has been assumed that total exposure, rather than pressure and time as independent variables, controls the types of adsorption structures formed. The importance of deposition pressure as a variable, however, was recently suggested in a study of the kinetics of the low pressure adsorption of oxygen on W(110) using X-ray photoelectron spectroscopy (XPS)and diffraction.@footnote 2@ As a more quantitative measure of such effects, we have used ultrahigh vacuum scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and XPS to explore the adsorbate structures and the coverage dependence of O/W(110) as a function of deposition pressure at room temperature. For the same total adsorbate exposure in Langmuirs (L), we find that changes in deposition pressure of as little as a factor of 3 cause significant changes in the apparent structures, domain sizes (e.g., of (1x2)O), and adsorbate coverage in monolayers. Total coverages derived from STM data using a software thresholding technique indicate that a critical dosing pressure exists for both the (2x1) and (2x2) structures, 1 x 10@sup -9@ and 3 x 10@sup -9@ torr respectively, below which coverage does not increase with increasing exposure time. This indicates that an equilibrium condition has been reached, from which surface free energies for both of these structures can be derived.@footnote 3@ Finally, in addition to the ordered (1x2), (2x2), and (1x1) structures for O/W(110), we have characterized a new fourth ordered structure by LEED and STM. The structure formed for oxygen exposure of 3 to 6 L and coexisted with the (1x2) structure. The primitive unit cell is a rectangle, 0.77nm x 1.37nm, with 15 W and 6 O atoms and has the matrix notation ((3,-1),(0,5)) relative to the W(110) substrate. @FootnoteText@ @footnote 1@ Supported by NSF DMR-9522240. @footnote 2@ Y. X. Ynzunza et al, Surf. Sci., in press. @footnote 3@ P. Liu et al, Surf. Sci. 417 (1998) 53.