High Resolution Electron Energy Loss Spectroscopy

Vibrational spectroscopy at metal surfaces

Motivation

"Most finely divided catalysts must have structures of great complexity. In order to simplify our theoretical consideration of reacqtions at surfaces, let us confine our attention to reactions on plane surfaces. If the principles in this case are well understood, it should then be possible to extend the theory to the case of porous bodies. In general, we should look upon the surface as consisting of a checkerboard..."

[Irving Langmuir, Trans. Faraday Soc. 17 (1922) 607]

This quotation by Irving Langmuir anticipatory describes the work of surface scientists since the sixties of the last century. Very successfully the interaction of a huge variety of adsorbates with low-indexed single crystal surfaces (Langmuir's "checkerboard") has been studied. By exploiting the high periodicity of these surfaces, processes could be explained in detail down to the atomic scale even before the scanning probe microscopy allowed imaging with atomic resolution in real space.

The second stage of Langmuir's vision, i.e. "to extend the theory to the case of porous bodies" is still work in progress: the difference between the "checkerboard" and the "porous bodies" (nowadays known as "material gap") is difficult to overcome. Obviously, the properties of rough defect-rich surfaces differ from those of smooth single crystal surfaces in many ways. An important field in which these differences are found is the heterogeneous catalysis.

Unfortunately, a lot of the established experimental methods fail when applied to rough surfaces. For this reason, it is necessary to find new experimental methods as well as useful preparation methods for surfaces within the material gap.

Preparing surfaces with defined defects

In order to examine the role of defects in the interaction with adsorbate molecules it is necessary to be able to produce well-defined defect sites. Our approach is to evaporate small amounts of metal on a single crystal surface consisting of the same metal. By varying the surface temperature during the evaporation process and by annealing the surface afterwards, we try to find well-defined and reproducible ways to prepare the defects.

Vibrational spectroscopy of metal surfaces

Recently, we found with high-resolution electron energy loss spectroscopy (HREELS) vibrational properties of small amounts of copper (below one monolayer) evaporated on a Cu(111) surface at extraordinary high energies (result to be published).

Vibrational spectroscopy of adsorbates on metal surfaces

The interaction of adsorbates with a defect-rich surface differs significantly from their interaction with smooth surfaces. We examined the interaction of ethylene with the roughened Cu(111) surface [1,2] by the use of HREELS and infrared reflection absorption spectroscopy (IRRAS). The interaction of carbon monoxide and oxygen with the roughened surface is currently under research. In addition to the vibration spectroscopy, we perform measurements of the work function change upon adsorption of molecules. For this purpose, we use the narrow energy distribution of the electrons from the HREELS gun to apply the potential retardation method.

Work function measurements

Using the highly monochromatic electron beam of the HREELS apparatus, work function changes of a sample surface can be measured with high precision. This gives additional information on i.e. morphology changes, adsorbate sites etc.

Collaboration and contact

If you are interested in further information or in a collaboration please contact Olaf Skibbe.

Literature:

[1]: O. Skibbe, M. Binder, A. Otto, and A. Pucci. Electronic contributions to infrared spectra of adsorbate molecules on metal surfaces: Ethene on Cu(111). J. Chem. Phys., 128:194703, 2008. [ DOI ]

[2]: O. Skibbe, D. Vogel, M. Binder, A. Pucci, T. Kravchuk, L. Vattuone, V. Venugopal, A. Kokalj, and M. Rocca. Ethene stabilization on Cu(111) by surface roughness. J. Chem. Phys., 131:024701, 2009. [ DOI ]

 
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Contact:
Dr. Olaf Skibbe
Phone: +49 6221 54 19890
olaf.skibbe@cam.uni-heidelberg.de
Im Neuenheimer Feld 225
69120 Heidelberg
Germany
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