THE EFFECT OF ANISOTROPIC MOLECULAR VIBRATIONS IN PHOTOELECTRON DIFFRACTION OF ADSORBED SPECIES

Anisotropic adsorbate vibrations can strongly influence the intensity modulation in energy-scan photoelectron diffraction. For adsorbed molecules the frustrated translations and rotations parallel to the surface are expected to be much softer than the vibrations of the molecule against the surface. In the present paper we therefore specifically include the effect of emitter vibrations in the multiple-scattering theory for photoelectron diffraction by means of a Debye-Waller factor. For the system NH3 on Ni{111} it is shown that the parallel vibrations have only a minor influence on the N 1s photoelectron diffraction spectrum in normal emission, whereas the intensity modulations in off-normal emission directions are damped considerably. A comparison between theory and experiment for the polar angles theta = 0-degrees, 25-degrees and 35-degrees, gives a value of 0.04 angstrom 2 for the mean-square displacement parallel to the surface, which corresponds to a tilt angle of 6-degrees.