Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?
The Journal of Physical Chemistry Letters
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Hedgeland, H., Sacchi, M., Singh, P., McIntosh, A., Jardine, A., Alexandrowicz, G., Ward, D., et al. (2016). Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?. The Journal of Physical Chemistry Letters, 7 (23), 4819-4824. https://doi.org/10.1021/acs.jpclett.6b02024
Mass transport at a surface is a key factor in heterogeneous catalysis. The rate is determined by excitation across a translational barrier and depends on the energy landscape and the coupling to the thermal bath of the surface. Here we use helium spin-echo spectroscopy to track the microscopic motion of benzene adsorbed on Cu(001) at low coverage (θ ∼ 0.07 ML). Specifically, our combined experimental and computational data determine both the absolute rate and mechanism of the molecular motion. The observed rate is significantly higher by a factor of 3.0 ± 0.1 than is possible in a conventional, point-particle model and can be understood only by including additional molecular (rotational) coordinates. We argue that the effect can be described as an entropic contribution that enhances the population of molecules in the transition state. The process is generally relevant to molecular systems and illustrates the importance of the pre-exponential factor alongside the activation barrier in studies of surface kinetics.
The authors are grateful for the support of the Leverhulme Trust (HH) and the Royal Society (MS). Via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work used the ARCHER UK National Supercomputing Service.
External DOI: https://doi.org/10.1021/acs.jpclett.6b02024
This record's URL: https://www.repository.cam.ac.uk/handle/1810/262308