Berlin 2005 – wissenschaftliches Programm

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O: Oberflächenphysik

O 30: Adsorption an Oberfl
ächen III

O 30.3: Vortrag

Montag, 7. März 2005, 11:15–11:30, TU EB301

Structural transition — •P. Tegeder, M. Danckwerts, S. Hagen und M. Wolf — Freie Universtät Berlin, Institut für

The adsorption behavior of 1,3,5,7-Cyclooctatetraene (COT) on Ru(001) from sub-monolayer coverages up to the multilayer regime has been studied by temperature-programmed desorption (TPD) and work function measurements. The electron dynamics at the COT/Ru(001) interface has been investigated using time- and angle-resolved two-photon photoemission (2PPE) spectroscopy. The TPD data show that COT films grow at 115 K in a metastable phase when the coverage is increased from the chemisorbed monolayer to the bulklike molecular multilayer structure. The metastable states desorb at a temperature which is ∼ 9 K lower than the multilayer, followed by an irreversible and thermally activated transformation into a stable multilayer phase. This transition occurs at 165±2 K and is accompanied by the appearance of image potential states and a pronounced increase of the total electron yield in 2PPE by more than one order of magnitude. The image states have binding energies of -0.70 eV and -0.24 eV for the n=1 and n=2 states, respectively, and a lifetime of 20±5 fs for both states. Their appearance is interpreted as an indication of island formation in the stable multilayer regime. 2PPE spectroscopy of the image potential states provides a sensitive probe of structural transitions in the adsorbate layers.