Münster 1997 – scientific programme

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O: Oberflächenphysik

O 16: Dynamik der Adsorption/Desorption

O 16.4: Talk

Tuesday, March 18, 1997, 10:15–10:30, PC 4

The Rotational degree of freedom in the Photodesorption of Ammonia from metal surfaces — •Marcello Binetti, Karl-Heinz Bornscheuer, Matthias Bittner, and Eckart Hasselbrink — Fritz–Haber–Institut der Max–Plank–Gesellschaft, Faradayweg 4–6, D-14195 Berlin

Ammonia photodesorption involves an internal molecular coordinate besides the ordinary distance between the centre of mass and the surface. The excitation of the adsorbate causes a transition from the pyramidal ground state to an electronically excited state of planar geometry. Hence the hydrogen atoms start adapting to the new geometry and are accelerated towards the surface. This motion continues also after the deexcitation and the resulting strong repulsion eventually causes the desorption of the molecule. During this process, the corrugation of the surface potential has an impact on the rotational energy. We have investigated this effect on Cu(111), Cu(100) and Ru(0001) surfaces. The energy content in this degree of freedom will also be discussed with special attention to the effect of hindrance of the free molecular rotation exerted by preadsorbed oxygen on Ru(0001) [1] . [1] C. Benndorf et al., Chem. Phys. Lett. 101 (1), 1983