Streuexperimente mit Wasserstoff- und Heliumstrahlen zur Untersuchung der Wechselwirkung von H2, N2 und C2H2 mit den (001)-Oberflächen von LiF, NaCl, KCl und MgO

Scattering experiments with molecular hydrogen and helium beams investigating the interactions of H2, N2 and C2H2 with the (001) surfaces of LiF, NaCl, KCl and MgO

von Franziska Traeger
Dissertation
Datum der mündl. Prüfung:2001-02-01
Erschienen:2001-09-24
Betreuer:Prof. Dr. Jan Peter Toennies
Gutachter:Prof. Dr. Jan Peter Toennies
Gutachter:Prof. Dr. Klaus Winzer
Zum Verlinken/Zitieren: http://dx.doi.org/10.53846/goediss-3584

 

 

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In the present work the interaction of hydrogen, nitrogen and acetylene with the surfaces of ionic crystals has been investigated. These studies use the scattering of molecular hydrogen beams from clean surfaces as well as information on structure and dynamics of molecular adsorbates obtained from elastic and inelastic helium atom scattering. In the scattering of H2 from the (001) surfaces of LiF, NaCl, KCl and MgO differences in the diffraction intensities between beams of normal-H2 ('n': 75 % ortho j = 1 und 25 % para j = 0) and para-H2 ('p': j = 0) have been observed. This effect can be attributed to the dependence of the electrostatic interaction of the molecular quadrupole moment with the gradient of the surface field on the orientation of the molecule. Helium diffraction from n-H2, p-H2, HD and D2 adsorbed on NaCl is consistent with (1 x 1) superstructures proposed in the literature. Time-of-flight spectra reveal several well resolved, dispersionless peaks, which can be assigned to a vibration parallel to the surface (7 meV for p-H2), anharmonic overtones (12 meV and 17 meV for p-H2) as well as double excitations of the fundamental vibrational mode. The external vibrational modes of o-H2 and p-H2 differ by about 1 meV, which again can be attributed of the different electrostatic interaction with the surface. Hydrogen adsorbed on MgO shows the series of c(2 x 2) - c(4 x 2) - c(6 x 2) structures as the coverage is increased followed by condensation of the second layer. Along the [110] direction two dispersionless modes have been measured for the c(4 x 2) phase at 8.5 and 10.5 meV, as well as a dispersive mode between 2 and 6 meV. The dynamics of the c(6 x 2) phase is similar. The second layer reveals only one dispersionless mode at around 5 meV. For comparison, the adsorbate N2/MgO has also been investigated. Acetylene adsorbed on KCl has been investigated between monolayer coverage and 40 layers. From diffraction measurements a (sqrt2 x sqrt2)R45° symmetry has been determined for the monolayer as well as for thicker layers. For the multilayers this symmetry corresponds to the orthorhombic modification of bulk acetylene. With inelastic helium atom scattering four rather strongly dispersive modes below 14 meV have been measured, which can be assigned to parallel, perpendicular and see-saw modes. Thicker layers display a Rayleigh mode with an energy of 6 meV at the zone boundary and three additional modes.
Keywords: molecular adsorbates; structure and dynamics; insulating surfaces; helium atom scattering; hydrogen molecular beam diffraction
 
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