AVS 56th International Symposium & Exhibition
    Surface Science Wednesday Sessions
       Session SS2-WeA

Paper SS2-WeA12
Activated Quantum Diffusion of Hydrogen on Platinum(111)

Wednesday, November 11, 2009, 5:40 pm, Room N

Session: Surface Physics, Single Particle Imaging
Presenter: A.P. Jardine, University of Cambridge, UK
Authors: A.P. Jardine, University of Cambridge, UK
E. Lee, University of Cambridge, UK
G. Alexandrowicz, University of Cambridge, UK
H.J. Hedgeland, University of Cambridge, UK
W. Allison, University of Cambridge, UK
J. Ellis, University of Cambridge, UK
Correspondent: Click to Email
Hydrogen atoms are one of the few surface species that are of sufficiently low mass for quantum processes to dominate surface transport. Here, we present the first quasi-elastic helium atomscattering (QHAS) measurements that demonstrate clear quantum effects in adsorbate diffusion. We use helium-3 spin-echo to make dynamic equilibrium measurements of the motion of H and D atoms on a Pt(111) surface, enabling lower temperatures to be studied than were possible with QHAS[1]. Our data shows a broken Arrhenius dependence, indicating two transport regimes; high temperatures (>120 K) and low temperatures (<110K). Otherwise, the data shows good agreement with a single jump model.

Our results offer a comprehensive dataset that is a severe test of theory. We compare the experimental data to existing protonic band structure calculations[2]. For H, it is possible to relate the measured activation energies to transitions to specific excited states of H, suggesting the diffusion of H on Pt(111) proceeds by activated quantum tunnelling. For D, the correspondance is less clear. We see a large change in pre-exponential factors with temperature, but not isotope, which we relate to energy exchange between adatoms and the surface. We compare the apparent rates of hopping with the expected tunnelling and energy transfer rates, in order to identify the rate limiting proceses.

Recent work suggests that surface steps determine the macroscopic behaviour. However, our measurements are sensitive to the local, microscopic behaviour and give an alternative picture of the quantum motion[3].

[1] A. P. Graham, A. Menzel and J. P. Toennies, J. Chem. Phys. 111, 1676 (1999).

[2] S. C. Badescu, P. Salo, T. Ala-Nissila, S. C. Ying, K. Jacobi, Y. Wang, K Bedurftig and G.Ertl, Phys. Rev. Lett. 88, 136101 (2002).

[3] C. Z. Zheng, C. K. Yeung, M. M. T. Loy, and Xudong Xiao. Phys. Rev. Lett. 97, 166101 (2006).