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We study the strong coverage dependence of the femtosecond laser-induced desorption of CO from Pd(111) using molecular dynamics simulations that consistently include the effect of the laser-induced hot electrons on both the adsorbates and surface atoms. Adiabatic forces are obtained from a multicoverage neural network potential energy surface that we construct using data from density functional theory calculations for 0.33 and 0.75 monolayer (ML). Our molecular dynamics simulations performed for these two trained coverages and an additional intermediate coverage of 0.60 ML reproduce well the peculiarities of the experimental findings. The performed simulations also permit us to disentangle the relative role played by the excited electrons and phonons on the desorption process and discover interesting properties of the reaction dynamics as the relevance that the precursor physisorption well acquires during the dynamics as coverage increases.

The photoinduced desorption and oxidation of CO on Ru(0001) is simulated using ab initio molecular dynamics with electronic friction that accounts for the nonequilibrated excited electrons and phonons. Different (O,CO) coverages are considered, the experimental room temperature coverage consisting in 0.5 ML-O + 0.25 ML-CO (low coverage), the saturation coverage achieved experimentally at low temperatures (0.5 ML-O + 0.375 ML-CO, intermediate coverage), and the equally mixed monolayer that is stable according to our calculations but not experimentally observed yet (0.5 ML-O + 0.5 ML-CO, high coverage). The results of our simulations for the three coverages are consistent with femtosecond laser experiments showing that the CO photodesorption largely dominates over CO photo-oxidation. These results cannot be explained in terms of the distinct activation energies calculated for the relaxed surfaces. Different (dynamical) factors such as the coupling to the laser-excited electrons and, more importantly, the interadsorbate energy exchange and the strong surface distortions induced in the more crowded surfaces are fundamental to understand the competition between these two processes under the extremely nonequilibrated conditions created by the laser.

244 p., Carbon monoxide (CO) is a neurotoxic gas emitted for instance in combustion reaction. Therefore it hasbeen sought for air treatment solution, where CO oxidation is a straight forward choice. In ultra highvacuum conditions the ruthenium has been found to be very inactive for CO oxidation. Experimentally ithas been shown the opening of a new reaction path for CO oxidation on ruthenium surfaces by means offem to second laser irradiation. Accurate simulations of the photo-reaction dynamics are required to give aproper characterization of this kind of experiments. This thesis is dedicated to the study of the photo induced desorption and oxidation of CO molecules, coadsorbed with oxygen (O) adatoms on Ru(0001)with different surface coverages. We began with the characterization of three (O, CO) mixed surface coverages on Ru(0001). We first found the adsorption configuration of minimum energy for each surface coverage, then we computed the desorption potential of a CO molecule, and found the minimum energy path to CO oxidation on all three surface coverages. Then we ran ab-initio molecular dynamics with electronic friction simulations, and we have been able to show the complexity of the reaction path to oxidize the CO molecule, and explain its low probability of occurrence. Next, we showed the importance of surface deformations on the desorption and oxidation probabilities of CO, and on the adsorbat emotion. Then, we have shown in detail and characterized the different mechanisms of CO oxidation.Finally we created a potential energy surface based on neural networks and showed that it is a very promising tool to solve the problem of the computational cost of ab-initio molecular dynamics simulations.

Carbon monoxide (CO) is a neurotoxic gas emitted for instance in combustion reaction. Therefore it hasbeen sought for air treatment solution, where CO oxidation is a straight forward choice. In ultra highvacuum conditions the ruthenium has been found to be very inactive for CO oxidation. Experimentally ithas been shown the opening of a new reaction path for CO oxidation on ruthenium surfaces by means offem to second laser irradiation. Accurate simulations of the photo-reaction dynamics are required to give aproper characterization of this kind of experiments. This thesis is dedicated to the study of the photo induced desorption and oxidation of CO molecules, coadsorbed with oxygen (O) adatoms on Ru(0001)with different surface coverages. We began with the characterization of three (O, CO) mixed surface coverages on Ru(0001). We first found the adsorption configuration of minimum energy for each surface coverage, then we computed the desorption potential of a CO molecule, and found the minimum energy path to CO oxidation on all three surface coverages. Then we ran ab-initio molecular dynamics with electronic friction simulations, and we have been able to show the complexity of the reaction path to oxidize the CO molecule, and explain its low probability of occurrence. Next, we showed the importance of surface deformations on the desorption and oxidation probabilities of CO, and on the adsorbat emotion. Then, we have shown in detail and characterized the different mechanisms of CO oxidation.Finally we created a potential energy surface based on neural networks and showed that it is a very promising tool to solve the problem of the computational cost of ab-initio molecular dynamics simulations.

Using many-body perturbation theory and density functional perturbation theory, we study the vibrational spectra of the internal stretch (IS) mode of CO on Pd(111) for the bridge and hollow adsorption structures that are experimentally identified at 0.5 ML coverage. Our theoretical treatment allows us to determine the temperature dependence of the IS vibrational spectra under thermal conditions as well as the time evolution of the nonequilibrium transient spectra induced by femtosecond laser pulses. Under thermal conditions (i.e., for equal electronic Te and phononic Tl temperatures), the calculated lifetimes at 10–150 K are mostly due to nonadiabatic couplings (NCs), i.e., first-order electronic excitations. As temperature increases, also the contribution of the second-order electron-mediated phonon-phonon couplings (EMPPCs) progressively increases from 25% at low temperatures to 50% at 300 K. Our calculations for the laser-induced nonequilibrium conditions comprise experimental absorbed fluences of 6–130J/m2. For fluences for which Te>2000K, the transient vibrational spectra are characterized by two different regimes that follow the distinct time evolution of Te and Tl and are respectively dominated by NC and EMPPC processes. At lower fluences, the initial fast regime becomes progressively negligible as Te decreases and only the steady second regime remains visible. Qualitatively, all these spectral properties are common to both of the adsorption structures studied here.

The role played by electronic and phononic excitations in the femtosecond laser induced desorption and oxidation of CO coadsorbed with O on Ru(0001) is investigated using ab initio molecular dynamics with electronic friction. To this aim, simulations that account for both kind of excitations and that only consider electronic excitations are performed. Results for three different surface coverages are obtained. We unequivocally demonstrate that CO desorption is governed by phononic excitations. In the case of oxidation the low statistics does not allow to give a categorical answer. However, the analysis of the adsorbates kinetic energy gain and displacements strongly suggest that phononic excitations and surface distortion also play an important role in the oxidation process.

Femtosecond laser-induced dynamics of molecules on metal surfaces can be seamlessly simulated with all nuclear degrees of freedom using ab initio molecular dynamics with electronic friction (AIMDEF) and stochastic forces, which are a function of a time-dependent electronic temperature. This has recently been demonstrated for hot-electron-mediated desorption of hydrogen molecules from a Ru(0001) surface covered with H and D atoms [Juaristi, J. I. Phys. Rev. B 2017, 95, 125439]. Unfortunately, AIMDEF simulations come with a very large computational expense that severely limits statistics and propagation times. To keep ab initio accuracy and allow for better statistical sampling, we have developed a neural network interatomic potential of hydrogen on the Ru(0001) surface based on data from ab initio molecular dynamics simulations of recombinative desorption. Using this potential, we simulated femtosecond laser-induced recombinative desorption using varying unit cells, coverages, laser fluences, and isotope ratios with reliable statistics. As a result, we can systematically study a wide range of these parameters and follow dynamics over longer times than hitherto possible, demonstrating that our methodology is a promising way to realistically simulate femtosecond laser-induced dynamics of molecules on metals. Moreover, we show that previously used cell sizes and propagation times were too small to obtain converged results.

In time-resolved pump-probe vibrational spectroscopy the internal stretch mode of polar molecules is utilized as a key observable to characterize the ultrafast dynamics of adsorbates on surfaces. The adsorbate nonadiabatic intermode couplings are the commonly accepted mechanisms behind the observed transient frequency shifts. Here, we study the CO/Pd(111) system with a robust theoretical framework that includes electron-hole pair excitations and electron-mediated coupling between the vibrational modes. A mechanism is revealed that screens the electron-phonon interaction and originates a blueshift under ultrafast nonequilibrium conditions. The results are explained in terms of the abrupt change in the density of states around the Fermi level, and are instrumental for understanding the dynamics at multicomponent surfaces involving localized and standard s or p states.