AVS 45th International Symposium
    Surface Science Division Thursday Sessions
       Session SS1-ThA

Paper SS1-ThA9
Novel Diffusion Channel Parallel to Steps: CO on Pt(111)

Thursday, November 5, 1998, 4:40 pm, Room 308

Session: Surface Diffusion
Presenter: X.D. Xiao, The Hong Kong University of Science and Technology, China
Authors: X.D. Xiao, The Hong Kong University of Science and Technology, China
J.-W. Ma, The Hong Kong University of Science and Technology, China
M.M.T. Loy, The Hong Kong University of Science and Technology, China
Correspondent: Click to Email
Surface diffusion parallel to steps for CO on Pt(111) has been studied by an optical diffraction technique. With a large accessible dynamic range for diffusion coefficient measurement from 10@super -12@ to 10@super -8@ cm@super 2@/sec, we could cover a wide range of temperatures from 120 to 320 K, within which non-Arrhenius processes can be unambiguously identified. For steps along [110] direction (B-type steps in the present experiment) with a density of one-step/30-atoms, it was found that diffusion at low temperatures are controlled by terrace diffusion and at high temperatures controlled by diffusion along step edges. At 0.3 ML CO coverage, the terrace diffusion was measured with an activation energy of 4.2 kcal/mol and a prefactor of 6x10@super -7@ cm@super 2@/sec. The diffusion along step edges was measured with an activation energy of 10.9 kcal/mol and a prefactor of 0.2 cm@super 2@/sec. It is this large prefactor that compensates the respect high activation barrier for step edge diffusion to take control at high temperatures. As we change the step direction to along [112], a novel diffusion channel was observed. Now, with the same step density as in the previous case, the diffusion is enhanced by a factor of ~10 over the entire temperature range as compared to the terrace diffusion. At 0.3 ML CO coverage, this new channel is characterized by a simple Arrhenius behavior with E@sub D@=3.2 kcal/mol and D@sub o@=5x10@super -7@ cm@super 2@/sec. At 0.5 ML CO coverage, the diffusion is dominated by this new channel only at low temperatures but still by along step edge diffusion at high temperatures. Discussion on how the new channel is formed will be presented.