Lüttgens, G. (Corresponding author)FZJ*

2002
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Report No.: Juel-3940

Abstract: Metal-adsorbate-clusters have been studied using femtosecond-time-resolved and conventional photoelectron spectroscopy. Information concerning the dissociation dynamics as well as the electronic and geometric structures of these systems could be deduced. The photodesorption of CO from Au$_{2}$CO- and Pt$_{2}$(CO)$_{5}$ after excitation with an 1.5 eV-photon could be studied with pump-probe photoelectron spectroscopy. The desorption occurs in a statistical manner via statistical fluctuations of the hot phonon system. The time constants of desorption are (470 ± 50) fs (Au$_{2}$CO$^{-}$) and (3 ± 2) ps (Pt$_{2}$(CO)$_{5}$). From the measured time constant one can deduce the energy barrier for CO-desorption. The photodissociation process of Pt$_{2}$N$_{2}$ occurs rather slowly on a time scale of $\tau$ = (100 ± 25) ps. The relaxation of the electronic system after excitation and the thermalisation of the excitation energy could be studied as well. The electronic relaxation in Au$_{2}$CO$^{-}$ and Pt$_{2}$(CO)$_{5}$ occurs very fast on an time scale < 70 fs. Using conventional photoelectron spectroscopy the electronic and geometric structures of platinum-, palladium and lead-dimmes with benzene-molecule adsorbed an them were studied. A strong interaction of the benzene-frontier-orbitals with the metal orbitals of the dimer can be deduced from the spectra. In the spectra of Pt$_{2}$Bz and Pd$_{2}$Bz a vibrational progression could be resolved. It could be concluded that the benzenemolecules adsorbs in an C$_{6v}$-like symmetry perpendicular to the axis of the dimer. Fitting a Franck-Condon-Profile to the vibrational progression the spectroscopy constants of Pt$_{2}$Bz, Pd$_{2}$Bz and the anions could be deduced. Contrary to this situation benzene was found to adsorb parallel to the main group metal dimer Pb$_{2}$ forming a Pb$_{2}$Bz-cluster with C$_{2v}$ symmetry.

Keyword(s): photoelectron spectroscopy ; metal cluster ; electronic structure

1. Elektronische Eigenschaften (IFF-9)