Analysis of thermal vibrations by temperature-dependent low energy electron diffraction: comparison of soft modes of pure and O-coadsorbed CO on Ru(0001)
Surface Science 441(1): 91-106
Extensive new temperature-dependent LEED I/V data of the well-known (root 3 x root 3)R30 degrees-CO structure and of the (2 x 2)-(O + CO) coadsorbate structure on Ru(0001) have been obtained down to 27 K and analysed in terms of the thermal vibrations of the CO molecule, both by tensor low energy electron diffraction (LEED) with isotropic vibrations and by a scheme based on probability density functions including anisotropic vibrations. In both structures the CO molecule occupies the top site, as has been shown by previous LEED I/V analyses. In the coadsorbate structure this top site is surrounded by three oxygen atoms on hcp sites and the molecule is tilted by about 13 degrees in a direction away from one of the oxygen atoms. The different treatments agree on all aspects of geometry and vibrations, and also with earlier results where overlap exists. We show that in the coadsorbate system the thermal vibrational amplitudes are much smaller than in the single adsorbate system, and that the tilt of the CO molecule found there is mainly of static nature. Around this axis the static lateral displacement of the oxygen atom of CO increases from 0.23 Angstrom at 27 K to 0.31 Angstrom at 350 K and the vibration amplitudes increase from 0.08 Angstrom to 0.11 Angstrom. The motion at the lowest temperature corresponds to the zero point motion. For the pure CO system the corresponding mean vibrational displacement of the O, which are now symmetric around the normal direction, range from 0.16 Angstrom at 27 K to over 0.4 Angstrom at 350 K. We conclude that low temperature measurements improve the accuracy of geometry determination by LEED I/V, and that temperature-dependent LEED can well be used for the determination of vibrational amplitudes. A corroboration of the reliability of LEED geometries is deduced from our extensive comparisons. (C) 1999 Elsevier Science B.V. All rights reserved.