title: Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?
abstract: 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.
authors.affiliation: School of Physical Sciences, The Open University , Walton Hall, Milton Keynes MK7 6AA, U.K.|Cavendish Laboratory, University of Cambridge , JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
authors.forename: Holly
authors.initials: H
authors.lastname: Hedgeland
authors.affiliation: Department of Chemistry, University of Surrey , Guildford GU2 7XH, U.K.
authors.forename: Marco
authors.initials: M
authors.lastname: Sacchi
authors.affiliation: The Perse School , Cambridge CB2 8QF, U.K.
authors.forename: Pratap
authors.initials: P
authors.lastname: Singh
authors.affiliation: Cavendish Laboratory, University of Cambridge , JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: McIntosh
authors.affiliation: Cavendish Laboratory, University of Cambridge , JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Jardine
authors.affiliation: Department of Chemistry, Technion - Israel Institute of Technology , Haifa 32000, Israel.
authors.forename: Gil
authors.initials: G
authors.lastname: Alexandrowicz
authors.affiliation: Cavendish Laboratory, University of Cambridge , JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
authors.forename: David J
authors.initials: DJ
authors.lastname: Ward
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, U.K.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Jenkins
authors.affiliation: Cavendish Laboratory, University of Cambridge , JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
authors.forename: William
authors.initials: W
authors.lastname: Allison
authors.affiliation: Cavendish Laboratory, University of Cambridge , JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
authors.forename: John
authors.initials: J
authors.lastname: Ellis