The mechanism and timescale of energy transfer from the substrate to the adsorbate is of basic interest for the understanding of elementary steps in heterogenious catalytical processes - as for example the oxidation of CO to CO₂ in automotive exhaust catalysts. This transfer occurs as consequence of interaction between the substrate electrons and phonons with the adsorbate on an ultrafast timescale, it can therefore be studied solely by optical techniques.

Topic of this thesis is the desorption and oxidation of CO over the (001)-face of a Ruthenium single crystal, which represents a well defined model system for surface reactions on transition metals. Experiments are carried out under ultra high vacuum conditions (UHV). They include fluence dependent and two pulse correlation measurements of the reaction yield after femtosecond laser excitation, time of flight spectroscopy of the reaction products, isotopic substitution techniques and a new technique for femtosecond time resolved vibrational spectroscopy by sum frequency generation (SFG). All experiments were set up in course of this work.

Analysis of the data is performed in two steps by standard models. The two temperature model provides a good description for the time evolution of the characteristic temperatures after substrate excitation by femtosecond laser pulses: the electronic system absorbs the photon energy and therefore reaches extreme transient temperatures which subsequently cooles down by coupling to the phonons. The validity of the model is demonstrated by the prediction of a dip in the surface phonon temperature in two pulse correlation experiments if both pulses overlap in time. This dip is caused by the competition between heat diffusion by electrons into the bulk (nonlinear in the electronic temperature) and energy transfer to the phonons (linear in the electronic temperature); it has been observed experimentally by pump-pump-reflectivity probe measurements [1]. The energy flow from the substrate to the adsorbate is treated by a friction-coupled heat bath model, consisting of the electronic and phononic System, which heat the adsorbate vibrationally to a temperature, that promotes the reaction. The mechanistic origin of the coupling constants is discussed both for the electronic and phononic interaction.

The CO desorption from CO/Ru(001) is determined to be driven by substrate phonons. Still - due to the extreme heating rates (1000 K/ps)- significant deviation of the reaction kinetics from equilibrium behaviour is found in the desorption probability and in the translational energy of the desorbing CO; these findings are discussed in a dynamical picture.[2]

By femtosecond laser excitation of the coadsorbate system CO/O/Ru(001) a novel reaction pathway is observed: the oxidation of CO to CO₂. By simple heating of the substrate this reaction is not possible, because the strongly bound atomic O can be regarded as inactive. When O is activated thermally for the oxidation, the weakly bound CO is already desorbed. However, the activation of O is found to be electronically driven and thus takes place prior to the phononically driven desorption of CO after femtosecond laser excitation, allowing the reaction to occur.[3] Therefore it is demonstrated, that differences in activation mechanisms can be exploited to control the outcome of a surface reaction by choosing the appropriate way of exciting the system.

A more direct manner of studying the energy transfer between the substrate and the adsorbate is provided by time resolved vibrational spectroscopy. Therefore a new SFG-probe scheme was implemented, utilizing a broadband IR pulse and a narrowband visible pulse to upconvert the complete IR spectrum to the visible where, by multi channel detection, it is detected simultaneously in a single laser shot. By this means, after exciting the system with an intense excitation pulse, transient spectra of the intramolecular C-O-vibration are obtained as a function of the delay between pump and probe. Preliminary results under excitation conditions leading to desorption are discussed. By time resolved SFG spectroscopy, where a broadband VIS-Pulse is time-delayed with respect to the IR-Pulse, the decoherence time of the vibration is determined directly in the time domain.

[1] M. Bonn, D. N. Denzler, S. Funk, M. Wolf, S.-S. Wellershoff, J. Hohlfeld. Ultrafast electron dynamics at metal surfaces: Competition between electron-phonon coupling and hot electron transport. Phys. Rev. B, expected 61, (2000), 1.

[2] S. Funk, M. Bonn, D. N. Denzler, Ch. Hess, M. Wolf, G. Ertl. Desorption of CO from Ru(001) induced by femtosecond laser pulses. submitted to J. Chem. Phys.

[3] M. Bonn, S. Funk, Ch. Hess, D. N. Denzler, C. Stampfl, M. Scheffler, M. Wolf, G. Ertl. Phonon- versus electron-mediated desorption and oxidation of CO on Ru(0001). Science 285, (1999) 1042.