Regensburg 2000 – wissenschaftliches Programm

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O: Oberflächenphysik

O 4: Adsorption an Oberflächen (I)

O 4.4: Vortrag

Montag, 27. März 2000, 12:00–12:15, H37

CO/Rh(111) revised by HREELS — •Ralf Linke and Johannes W. Niemantsverdriet — Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven

A new UHV system containing HREELS, TPD and LEED has been built up recently in our group. The HREELS features a resolution on the order of 2meV at high count rates and enables measurement of isotopic mixtures as well as resolution of the CO-metal vibrations of the different adsorption sites. At low coverage CO adsorbs only on-top. At 0.33ML a sharp (root3xroot3)R30 LEED pattern is observed. The vibrational frequencies in this structure are found at 253.1meV and 58.3meV for the CO stretching and CO-M stretching vibration, respectively. At about a half monolayer a second adsorption site begins to become occupied. At saturation coverage, indicated by a (2x2)-3CO LEED pattern, the respective vibrational frequencies are found at 230.8meV and 48.4meV and are assigned to CO adsorption in 3-fold hollow sites. The CO stretching vibration of the on-top site shifts towards 256.7meV. Upon the emergence of the second adsorption site a red shift of the on-top CO-M vibration is observed, reaching 53.8meV at saturation coverage. The observed coverage dependent shifts are discussed in terms of static and dynamic shifts. The vibrational spectra of isotopic mixtures confirm that shifts of the CO stretching vibration arise purely from dipole-dipole coupling.