Interactions of Atoms and Molecules with Insulator Surfaces

 

The investigation of insulating crystal surfaces using the standard surface science techniques is strongly hampered, because in most cases these methods employ electrically charged probing particles or leave the surface in an electrically charged state by removing ions or, in the case of photoemission experiments, electrons.
Many processes which are of importance in chemical technology and atmospheric chemistry on the other hand demand a better insight into elementary interactions of atoms and molecules with such systems. Helium atom scattering is a particularly useful tool to study insulating surfaces. The projectiles are not charged and, due to their small energies between 10 and 50 meV, are genuinely non-destructive.

One of the goals in the work of this group is to study the behavior and properties of molecular adsorbate layers on the surfaces of ionic crystals. By employing elastic and inelastic helium scattering one obtains information about the symmetry of the adsorbate phases, the vibrations of a completed layer or single adsorbed molecules, and on their binding energies. Further information can be gained from low energy electron diffraction (LEED) and Fourier transform infrared spectroscopy (FTIRS). The latter work is performed in laboratories at the University of Magdeburg by the group of H.Weiß. The interpretation of the experiments has been supplemented by theoretical work on the interaction potentials by other groups.

Recently experiments were carried out on molecular hydrogen adsorbed on the surfaces of LiF, NaCl, and MgO. Because of the low temperatures (7 K) needed to bind the hydrogen molecules and their low mass, many properties of the adsorbate phases can only be understood in terms of quantum mechanical phenomena. If one compares layers comprising molecules in the rotational state j=1 with those in which the molecules are in the rotational ground state j=0, one finds that, while the adsorbate structures and the registry to the substrate are the same, the vibrations with respect to the substrate are different. The reasons are differences in the electrostatic interactions with the differently charged ions of the surface. These forces are sensitive to whether or not the molecules are rotating about a fixed axis.

Figure 1 illustrates this situation and shows that the influence of attractive (red arrows) and repulsive (blue arrows) forces between the differently charged ions and the charge distribution on the molecule depends distinctively on the rotational state and the orientation of the axis of rotation with respect to the surface. This picture also shows, that the apparent surface corrugation experienced by the molecule depends on its rotational state.

We have also investigated the adsorption of water- and acetylene molecules as well as N2, CO, CO2, and OCS on various substrates like NaCl, MgO, LiF, KCl.

Information on the molecule - substrate interaction can also be found from a scattering experiment in which the molecules are scattered from the clean substrate surface. The dependence of the surface corrugation on the rotational state of the projectile sketched above leads to differences in the diffraction patterns for molecules in different rotational states. This effect, first observed in this laboratory, can be used to determine local electric fields or the charge distribution on the surface.


Fig.1
The cartoon illustrates how the attractive (arrows pointing down) and repulsive (arrows pointing up) electrostatic forces between the ions on a LiF surface and a hydrogen molecules lead to a situation in which the molecules in different rotational states experience different apparent corrugations of the surface.



For a collection of recent publications from 2004 to Sept. 2007 on surface scattering by this group please click here

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last revision:   E. Hulpke, October 2007