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O: Fachverband Oberflächenphysik

O 34: Metal Substrates: Adsorption of Organic/Bio Molecules II

O 34.3: Vortrag

Dienstag, 26. Februar 2008, 14:15–14:30, MA 043

Entropy driven stabilization of oligopyridine mono- and multilayer phases on HOPG - a thermal desorption study — •Michael Roos, Harry E. Hoster, and R. Jürgen Behm — Institute of Surface Chemistry and Catalysis, Ulm University

As shown in recent STM investigations, bis(terpyridine)derivates (BTP) form highly ordered hydrogen bonded networks on HOPG at both the solid/liquid [1] and the solid/gas [2,3] interface. In this study, we systematically investigate and compare the thermal desorption behavior for varying initial coverages of BTP molecules. We can clearly distinguish between two peaks corresponding to the mono- und multilayer regime. Surprisingly, the activation energies for desorption are about equal for both phases. Therefore, the different stabilities of mono- and multilayers must be associated with large differences in the preexponential factors, which are ν = 1·10²⁴ s⁻¹ for the zero order-like desorption from the multilayer and ν = 1·10¹⁸ s⁻¹ for the submonolayer regime. In this coverage range, we find a repulsion-dominated desorption behaviour, indicated by a pronounced down-shift of the onset of desorption, which is in contradiction to the expected attractive influence of the increasing number of C-H···N bonds. The thermal desorption results can be explained in terms of transition state theory, keeping in mind the large moment of inertia of the BTP molecules.

1. C. Meier et al., J Phys Chem B 109, 21015 (2005).

2. H. E. Hoster et al., Langmuir 23, 11570 (2007).

3. A. Breitruck et al., Surf Sci 601, 4200 (2007).