Analysis of thermal vibrations by temperature-dependent low energy electron diffraction: comparison of soft modes of pure and O-coadsorbed CO on Ru(0001)

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 el ectron diffraction (LEED) with isotropic vibrations and by a scheme based o n probability density functions including anisotropic vibrations. In both s tructures the CO molecule occupies the top site, as has been shown by previ ous LEED I/V analyses. In the coadsorbate structure this top site is surrou nded 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 syste m the thermal vibrational amplitudes are much smaller than in the single ad sorbate 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 o xygen atom of CO increases from 0.23 Angstrom at 27 K to 0.31 Angstrom at 3 50 K and the vibration amplitudes increase from 0.08 Angstrom to 0.11 Angst rom. The motion at the lowest temperature corresponds to the zero point mot ion. 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 t emperature measurements improve the accuracy of geometry determination by L EED I/V, and that temperature-dependent LEED can well be used for the deter mination of vibrational amplitudes. A corroboration of the reliability of L EED geometries is deduced from our extensive comparisons. (C) 1999 Elsevier Science B.V. All rights reserved.