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Abstract

This thesis investigates the reaction mechanisms of femtosecond-laser induced associative desorption from metal surfaces. The energy transfer between a metal substrate and an adsorbate occurs on an femtosecond (fs) timescale, mediated by coupling to the phonons and the electrons. The potential energy surfaces quantifying the forces between the atoms involved in a reaction are usually derived under the assumption that the electrons follow instantaneously the nuclear motion of the atoms. This is named the adiabatic or Born-Oppenheimer approximation. Thereby, non-adiabatic coupling effects between nuclear motions and electronic degrees of freedom are neglected. The applicability of the Born-Oppenheimer approximation to reactions on metal surfaces is a topic of intense debate, due to the possibility of low-energetic electron-hole pair excitations in the metal substrate. The importance of nonadiabatic, i.e. electronic, contributions are studied with fs-laser pulses exploiting the strong non-equilibrium between electrons and phonons directly after excitation.
In this thesis, the fs-laser induced reactions Hads + Hads -> H2,gas and Cads + Oads -> COgas on Ru(001) are examined. The analysis of the experimental data is based on the two-temperature model, describing the temporal evolution of the electron and phonon temperatures after excitation by fs-laser pulses, and frictional coupling between adsorbate and substrate.
For the purely electron mediated ultrafast hydrogen recombination the energy transfer to different degrees of freedom of the desorbing molecule has been examined by performing resonance enhanced multiphoton ionization (REMPI) and time-of-flight (TOF) measurements. Unequal energy partitioning is found with a ratio of 2.7:1.3:1 for translational, vibrational and rotational energies expressed in terms of the corresponding temperatures. Ab initio molecular dynamic calculations considering electronic coupling performed by Luntz et al. [1] reveal that the energy partitioning is due to the topology of the adiabatic potential energy surface and not due to anisotropic electronic coupling.
The associative desorption of CO is found to be driven by both substrate electrons and phonons which results in an ultrafast reaction mechanism although the width of the measured two-pulse correlation is ~ 20 ps, which is usually interpreted as evidence for purely phonon mediated reactions. The determined electronic coupling strength is comparable with theoretical predictions for related systems and one order of magnitude larger than for H2/Ru(001). Excitation with 400 nm instead of 800 nm light enhances the reaction rate, an effect which is due to the shorter optical penetration depth for 400 nm light causing higher surface temperatures. The translational energy of the desorbing CO is found to be lower than expected for desorption under equilibrium conditions which might be due non-adiabatic damping or unequal energy partitioning.
In summary, it is found that non-adiabatic electronic coupling is dominating the activation of both investigated recombination processes. The energy partitioning in the associative desorption of hydrogen is governed by the topology of the adiabatic ground state.
Ref.: [1] Luntz et al., J. Chem. Phys. 124, (2006) 244702.