IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM1
A Molecular Beam Study of Water Adsorption, Desorption, and Clustering on Pt(111)

Tuesday, October 30, 2001, 8:20 am, Room 122

Session: Water at Surfaces
Presenter: J.L. Daschbach, Pacific Northwest National Laboratory
Authors: J.L. Daschbach, Pacific Northwest National Laboratory
B.M. Peden, Pacific Northwest National Laboratory
G. Teeter, Pacific Northwest National Laboratory
R.S. Smith, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
Correspondent: Click to Email
Adsorption, desorption, and clustering are investigated by molecular beam techniques. Specular He atom scattering is used to probe the sub-monolayer H2O surface coverage on Pt(111) over the temperature range 22K to 185K. Structural rearrangements of H2O adsorbed at 22K are studied as a function of initial coverage and temperature in the coverage range from 0.01 ML to 1.0 ML by measuring the specular He intensity using linear temperature ramps in the range 0.1 K/s to 10 K/s. Either or both of two transitions are observed depending on coverage, with the first corresponding to the onset of surface diffusion and trapping of mobile H2O by step edges or defects, and the second corresponding to Oswald ripening of 2-D condensed phase islands. Adsorption and desorption kinectics are interrogated isothermally by measuring the H2O coverage in time as a function of H2O beam flux and temperature. At temperatures between 150 K and 165 K the adsorption and desorption spectra are linear in time and therefore independent of coverage. In this region the desorption kinetics are strictly zero-order and can be measured with high precision. The zero order kinetics are a consequence of the existence of a 2-D two phase H2O system present on the Pt surface. At temperatures above 172 K, depending on flux, a transition to non-zero order kinetics is observed with the kinetics consistent with first order. This transition occurs when the system has moved from a two-phase coexistence region to a single phase 2-D gas.