Heating of a ruthenium surface on which carbon monoxide and atomic oxygen are coadsorbed leads exclusively to desorption of carbon monoxide. In contrast, excitation with femtosecond infrared laser pulses enables also the formation of carbon dioxide. The desorption is caused by coupling of the adsorbate to the phonon bath of the ruthenium substrate, whereas the oxidation reaction is initiated by hot substrate electrons, as evidenced by the observed subpicosecond reaction dynamics and density functional calculations. The presence of this laser-induced reaction pathway allows elucidation of the microscopic mechanism and the dynamics of the carbon monoxide oxidation reaction.

Using time-resolved sum-frequency generation spectroscopy, the C-O stretch vibration of carbon monoxide adsorbed on a single-crystal Ru(001) surface is investigated during femtosecond near-IR laser excitation leading to desorption. A large transient redshift, a broadening of the resonance, and a strong decrease in intensity are observed. These originate from coupling of the C-O stretch to low-frequency modes, especially the frustrated rotation, that are highly excited in the desorption process. PACS numbers: 68.35.Ja, 33.70.Jg, 78.47. + p, 82.20.Rp The dynamics of the interaction between molecules and metal surfaces is of fundamental importance in surface science, since these determine key physical and chemical properties-essential in, e.g., catalysis-such as energy transfer and molecular reactivity [1] [6] at surfaces. These experiments were limited to temperatures below those at which desorption of the adsorbate occurs. However, when one is interested in surface reactions, the more relevant situation occurs at higher temperatures, where higher-lying vibrational modes get thermally occupied that play an important role in the surface chemistry.We present here femtosecond time-resolved vibrational sum-frequency generation (fs-SFG) spectra of CO molecules adsorbed on a Ru(001) surface, taken while a significant number (ϳ50%) of these molecules is desorbing due to fs laser excitation. With this fs-SFG method, snapshots of the (C-O) stretch vibration can be taken, under conditions where desorption is occurring (at lattice temperatures transiently exceeding the desorption temperature by over 500 K), shedding new light on the dynamics of the desorption process and the coupling of vibrational modes at the surface.The experiments were carried out in an ultrahigh vacuum chamber (base pressure 1 3 10 210 mbar) equipped with standard surface science tools. Our commercial laser system produces 800 nm, 110 fs pulses of 4.5 mJ͞pulse at 400 Hz, chopped down to 20 Hz for these experiments. One-third of the energy is used to excite the surface ("pump" pulse, typical absorbed fluence 55 J͞m 2 [7]), and the remaining energy is used to pump an optical parametric generator/amplifier (OPG͞OPA) providing tunable (l 2 10 mm, bandwidth ϳ150 cm 21 ) IR pulses with an energy of typically 10 mJ and a duration of 150 fs. The portion of the 800 nm pulse which is not converted into IR in the OPG͞OPA-process is spectrally narrowed down to 4 cm 21 , and is used in the SFG experiments as the visible (VIS) up-conversion pulse. An extensive description of the complete experimental setup as well as the surface cleaning, preparation, and characterization procedures can be found in Ref. [8].The surface sensitivity of SFG relies on the fact that the second-order nonlinear susceptibility is nonvanishing at interfaces [9]. This enables the generation of an electric field E SFG out of two incidents fields E VIS and E IR , where energyhv and parallel momentum k k must be con-For frequencies v IR within the IR bandwidth resonant with the vibrational tra...

Irradiation of a Ru͑001͒ surface covered with CO using intense femtosecond laser pulses ͑800 nm, 130 fs͒ leads to desorption of CO with a nonlinear dependence of the yield on the absorbed fluence ͑100-380 J/m 2 ͒. Two-pulse correlation measurements reveal a response time of 20 ps ͑FWHM͒. The lack of an isotope effect together with the strong rise of the phonon temperature ͑2500 K͒ and the specific electronic structure of the adsorbate-substrate system strongly indicate that coupling to phonons is dominant. The experimental findings can be well reproduced within a friction-coupled heat bath model. Yet, pronounced dynamical cooling in desorption, found in the fluence-dependence of the translational energy, and in a non-Arrhenius behavior of the desorption probability reflect pronounced deviations from thermal equilibrium during desorption taking place on such a short time scale.

An experimental scheme ͑double pump/reflectivity probe using femtosecond laser pulses͒ enables the investigation of nonequilibrium electron dynamics at metal surfaces by measuring the equilibrated surface temperature. The competition between electron-phonon coupling and hot-electron transport gives rise to a reduced equilibrated temperature when the two pump pulses overlap in time, and provides a way of accurately determining the electron-phonon coupling constant. These observations have important consequences for femtosecond photochemical investigations.

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Fundamental insights into the ultrafast dynamics of energy transfer processes at surfaces are of central importance for a microscopic understanding of chemical reactions at solid surfaces, e.g. in heterogeneous catalysis. The detailed investigation of the rates and pathways of energy flow in the adsorbate-substrate system as well as the chemical dynamics has become possible utilizing intense femtosecond laser pulses. Surprisingly, the strong non-equilibrium situation that arises upon irradiation with these pulses results in a new reaction channel caused by hot electron excitation for a number of systems investigated. This electron-mediated reaction mechanism is exemplified here for the model reaction H ad +H ad → H 2,g on Ru(001) and discussed in the broader context of other simple reactions such as the formation of H 2 O, CO 2 and the CO desorption on the same surface.