AVS 53rd International Symposium
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM6
Rotational Disorder in Ordered Overlayer Phases

Wednesday, November 15, 2006, 9:40 am, Room 2004

Session: Electronic and Vibrational Excitations and Dynamics
Presenter: E.Z. Ciftlikli, Rutgers, State University of New Jersey
Authors: E.Z. Ciftlikli, Rutgers, State University of New Jersey
I.G. Shuttleworth, Rutgers, State University of New Jersey
A.V. Ermakov, Rutgers, State University of New Jersey
J. Lallo, Rutgers, State University of New Jersey
S. Rangan, Rutgers, State University of New Jersey
S.D. Senanayake, Oak Ridge National Laboratory
B.J. Hinch, Rutgers, State University of New Jersey
Correspondent: Click to Email
CN containing moieties, such as C@sub 2@N@sub 2@, CN@super -@, CH@sub 3@CN and NCO species, are frequently observed as reactants, products and intermediates in many industrial heterogeneous catalytic processes. Despite their importance, there are relatively few published reports on the structure and dynamics of these species on metallic fcc close packed faces. It may be that electron and/or photon induced damage has hindered many such studies. Helium atom scattering (HAS), employing thermal energy atoms of E<100 meV, is free of any detrimental/destructive effects. For example, angle resolved HAS measurements clearly show the existence of c(10x6) and c(2x2) ordered structures for CN@super -@/Cu(001) and CH@sub 3@CN/Cu(001) surfaces, respectively. And yet energy resolved HAS measurements, which are designed to probe the low-energy vibrational modes of the bound moieties, do not indicate discrete low energy features associated with of localized modes of the adsorbates. Broad multiphonon-like features dominate the TOF spectra, in both systems, and neither discrete frustrated translations, nor frustrated rotational modes are observed. Two contributions are apparent in the Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra, suggesting that there are at least two CN species in the CN@super -@/Cu(001) system. One has horizontal alignment, the other shows no preferred spatial orientation. The NEXAFS measurements from the CH@sub 3@CN/Cu(001) system suggest that there is only one type of adsorbate species, showing no preferred orientation. The lack of orientation observed in those ordered systems can imply that the surface moieties have a considerable degree of rotational freedom. DFT calculations have predicted low rotational barriers for adsorbed CN@super -@/fcc(001) systems. We discuss the helium scattering, as well as the NEXAFS spectra and analysis, in the light of this model for the dynamic adsorbate systems.