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Hydrogen Dynamics

The helium­3 surface spin echo technique is ideally suited to fundamental investigations into hydrogen dynamics at surfaces. The main focus of the experiments so far has been on diffusion. The spin echo experiment reveals information about the mechanism of diffusion on atomic lengthscales and timescales. The physical issues we are addressing include the H/surface potential energy landscape and energy exchange, single and multiple jumps in the diffusion process, the role of quantum tunnelling in hydrogen diffusion, and isotope effects. The results are benchmarks for semiclassical and quantum theories of diffusion rates. The experiments are also relevant to the study of chemical reactions at  surfaces since we are directly measuring the rate of elementary reaction steps.

To date, published spin echo data refer to the diffusion of protium (1H) and deuterium (2H) on the Pt(111) surface [1] and the diffusion of protium on the Ru(0001) surface [2]. Together the above investigations show that the influence of nuclear quantum mechanical effects on hydrogen diffusion depends strongly on the details of the potential energy landscape. The existence two nearly­degenerate adsorption sites per unit cell for H on the Ru(0001) surface allows for fast incoherent tunnelling; the same process on Pt(111) is not a significant part of the dynamics as the 'second site' is too high in energy to play a significant role in the quantum dynamics.

A primary aim of further hydrogen diffusion measurements has been to clarify the above incoherent tunnelling interpretation. Another area that HeSE is uniquely placed to address is the ultra­fast (sub­picosecond) equilibrium dynamics of diffusing hydrogen atoms, the relationship between the ultra­fast motion and diffusion, nuclear quantum­mechanical effects on very short timescales and the relationship to the H/surface bandstructure.

Hydrogen dynamics remains a very active area of research in the Surface Physics division of SMF. Expect data for many other surfaces to emerge soon!

[1] A. P. Jardine, E. Y. M. Lee, D. J. Ward, G. Alexandrowicz, H. Hedgeland, W. Allison, J. Ellis, and E. Pollak. "Determination of the Quantum Contribution to the Activated Motion of Hydrogen on a Metal Surface: H/Pt(111)" Physical Review Letters, 105(13):136101, 2010

[2] Eliza M. McIntosh, K. Thor Wikfeldt, John Ellis, Angelos Michaelides, and William Allison "Quantum effects in the diffusion of hydrogen on Ru(0001)" The Journal of Physical Chemistry Letters, 4(9):1565–1569, 2013.s