AVS 51st International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP45
Competing Kinetic and Energetic Effects in the Adsorption of Ortho- and Para-H@sub 2@ on NaCl(001)

Tuesday, November 16, 2004, 4:00 pm, Room Exhibit Hall B

Session: Poster Session
Presenter: F. Traeger, Ruhr-Universität Bochum and MPISF Göttingen, Germany
Authors: J.P. Toennies, Max-Planck-Institute für Strömungsforschung, Germany
F. Traeger, Ruhr-Universität Bochum and MPISF Göttingen, Germany
C. Wöll, Ruhr-Universität Bochum, Germany
Correspondent: Click to Email
Due to the electrostatic interaction of the hydrogen molecule's quadrupole moment with the electric field of the surface the ortho and para modifications experience slightly different molecule-surface potentials, which has been verified by scattering of n- and p-H@sub 2@ beams from the clean NaCl surface. A complementary way to study the molecule-surface interaction is to investigate the (1x1) monolayers of n-H@sub 2@ and p-H@sub 2@ on NaCl at 8 K with elastic and inelastic He atom scattering. The ortho-para differences result in 10-12 % higher frequencies for the external vibrations of o-H@sub 2@. Therefore, the relative intensities of the peaks in time-of-flight spectra of the scattered He atom provide information on the composition of the layer. Under steady-state conditions at H@sub 2@ gas pressures between 2.5 x 10@super -8@ mbar and 9.3 x 10@super -7@ mbar as well as during the desorption of the layer without gas pressure time-of-flight spectra have been recorded and are compared to desorption curves, i.e. the specular He intensity during desorption. Conflicting evidence regarding the adsorption probability for ortho- and para-H@sub 2@ is found, which is only partly in agreement with a model of thermodynamically and kinetically controlled adsorption proposed by Heidberg et al. from IR spectroscopy.