Search

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.

CO oxidation on Ru(0001) is a long-standing example of a reaction that, being thermally forbidden in ultrahigh vacuum, can be activated by femtosecond laser pulses. In spite of its relevance, the precise dynamics of the photoinduced oxidation process as well as the reasons behind the dominant role of the competing CO photodesorption remain unclear. Here we use ab initio molecular dynamics with electronic friction that account for the highly excited and nonequilibrated system created by the laser to investigate both reactions. Our simulations successfully reproduce the main experimental findings: the existence of photoinduced oxidation and desorption, the large desorption to oxidation branching ratio, and the changes in the O K-edge X-ray absorption spectra attributed to the initial stage of the oxidation process. Now, we are able to monitor in detail the ultrafast CO desorption and CO oxidation occurring in the highly excited system and to disentangle what causes the unexpected inertness to the otherwise energetically favored oxidation.

Two-dimensional transition-metal dichalcogenides appear as good candidates for gas sensing and catalysis. Here, by means of density functional theory, we characterize the adsorption and dissociation of selected diatomic molecules (CO, H2, O2, and NO) on he MoSe2 monolayer. We consider that these processes occur on the pristine 1H−MoSe2 monolayer and in the vicinity of an isolated Se vacancy. The presence of a single Se vacancy both enhances the molecular adsorption and reduces the energy needed for dissociation, making it energetically favorable for the case of O2 and NO molecules. Moreover, the presence of a second Se vacancy makes the dissociation process energetically favorable for all the molecules that are studied here. For each case we evaluate the effect that each adsorbate has on the electronic structure of the MoSe2 monolayer and the charge transfer that takes place between the adsorbate and the surface. Remarkably, adsorption of either CO or NO at the Se vacancy induces a finite spin-magnetization in the system that is spatially well localized around the adsorbate and the three closest Mo atoms.

Despite the great success of density functional theory in describing materials, there are still a few examples where current exchange-correlation functionals fail. We add another example to this list that drives further development of functionals. We show that the interaction of O2 with Ag(110) cannot be properly described by some of the most popular GGA, meta GGA, and hybrid functionals. We identify problems and provide clues for a functional that should be able to describe this and similar systems properly.

Neural network-based potential energy surfaces are currently gaining relevance in the context of gas-solid dynamics. In this work, we use this methodology to theoretically explore femtosecond laser pulse induced desorption of CO from the Pd(111) surface with a coverage of 0.75 ML. We performed molecular dynamics simulations in the high laser fluence regime on two different CO isotopes, 12C16O and 13C18O, in order to search for possible isotope effects affecting the photo-induced desorption. According to our findings, isotope effects of relevance do not appear in desorption probabilities once the whole process has finished after 50 ps. However, for those molecules desorbed with the highest translational kinetic energies, we obtain that 12C16O molecules are more vibrationally excited than 13C18O. This mostly happens as a consequence of the interaction of adsorbed CO with the laser-excited electrons in the substrate.

Femtosecond laser induced desorption of CO from a CO-covered Pd(111) surface is investigated with ab initio molecular dynamics with electronic friction that incorporates effects due to the excited electronic and phononic systems, as well as out-of-phase coadsorbate interactions. Our simulations show evidence of an important electron-phonon synergy in promoting CO desorption that has largely been neglected in other similar systems. At the saturated coverage of 0.75 ML, effects due to CO-CO interadsorbate energy exchange are also important. Our dynamics simulations, in concert with site-specific desorption energy calculations, allow us to understand the large coverage dependence of the desorption yields observed in experiments.

Measured lifetimes of the CO internal stretch mode on various metal surfaces routinely lie in the picosecond regime. These short vibrational lifetimes, which are actually reproduced by current first-principles nonadiabatic calculations, are attributed to the rapid vibrational energy loss that is caused by the facile excitation of electron-hole pairs in metals. However, this explanation was recently questioned by the huge discrepancy that exists for CO on Au(111) between the experimental vibrational lifetime that is larger than 100 ps and the previous theoretical predictions of 4.8 and 1.6 ps. Here, we show that the state-of-the-art nonadiabatic theory does reproduce the long CO lifetime measured in Au(111) provided the molecule-surface interaction is properly described. Importantly, our new results confirm that the current understanding of the adsorbates' vibrational relaxation at metal surfaces is indeed valid.

We present ab initio molecular dynamics simulations based on the density functional theory to study the alignment-dependent scattering of O from highly oriented pyrolytic graphite (HOPG). Results are obtained for 200 meV O molecules with different alignments impinging with an angle of incidence measured from a surface normal of 22.5° on a thermalized (110 and 300 K) graphite surface. The choice of these initial conditions in our simulations is made to perform comparisons with recent experimental results on this system. Our results show that a lower number of molecules initially aligned normally to the surface is reflected in the scattering plane compared to the number of molecules initially aligned parallel to the surface. This is not due to an increase in the trapping of end-on-aligned molecules but due to the fact that a large amount of them are scattered out-of-plane. Additionally, we find that the final translational energy of end-on molecules is around 10% lower than that of side-on molecules. We show that this is due to a more efficient energy transfer from the translational degree of freedom to internal degrees of freedom for end-on molecules, and not due to a more efficient energy transfer to the surface. The results of our simulations are in overall agreement with the experimental observations regarding the alignment dependence of the in-plane scattering probabilities and the energy loss of the reflected molecules.

The oxidation of carbon monoxide (CO) on different transition metal surfaces, including ruthenium, is a benchmarkreaction for theoretical and experimental studies of heterogeneous catalysis. Bonn and coworker investigated the photo-induced desorption andoxidation of CO adsorbed on an oxygen pre-covered surface of Ru(0001) with a 800nm femtosecond laser. This experiment predicts that, theelectronic excitations created by the laser pulse modify the yield of the reaction by promoting the recombination of carbon monoxide and oxygenatom. Simulations based on the advanced ab-initio molecular dynamics method that includes electronic friction (AIMDEF) and thermostats showthat not only the laser-excited electrons matter. It is the electron-phonon synergy that ultimately facilitates the CO oxidation on the surface.

A novel approach based on a neuralnetwork (NN)-generated potential energy surface (PES) is developed to describe the dynamics of the femtosecond laser-induced desorption of COfrom Pd(111). Using trajectories computed with (Te,Tl)ab-initiomolecular dynamics with electronic friction (AIMDEF) as input data, the NN-PES istrained within the embedded atom neural network framework using the atomic configurational energies and forces. The NN-PES robustness ischecked by studying the errors in energies and forces, and also by testing its performance in complex molecular dynamics simulations. The (Te,Tl)-AIMDEF results are reproduced with a remarkable level of accuracy. This shows the outstanding performance of the obtained NN-PES that cancover an extensive range of surface temperatures (90-1000 K) and a large amount of degrees of freedom -those corresponding to multipleadsorbates and surface atoms. Application of this NN-PES for future computational tests of the system dynamics under different initial conditionsshould be straightforward, as well as the utilization of this methodological framework for development of accurate NN-PESs for other complex gas-solid interfaces.

A high dimensional and accurate atomistic neural network potential energy surface (ANN-PES) that describes the interaction between one O2 molecule and a highly oriented pyrolytic graphite (HOPG) surface has been constructed using the open-source package (aenet). The validation of the PES is performed by paying attention to static characteristics as well as by testing its performance in reproducing previous ab initio molecular dynamics simulation results. Subsequently, the ANN-PES is used to perform quasi-classical molecular dynamics calculations of the alignment-dependent scattering of O2 from HOPG. The results are obtained for 200 meV O2 molecules with different initial alignments impinging with a polar incidence angle with respect to the surface normal of 22.5° on a thermalized (110 and 300 K) graphite surface. The choice of these initial conditions in our simulations is made to perform comparisons to recent experimental results on this system. Our results show that the scattering of O2 from the HOPG surface is a rather direct process, that the angular distributions are alignment dependent, and that the final translational energy of end-on molecules is around 20% lower than that of side-on molecules. Upon collision with the surface, the molecules that are initially aligned perpendicular to the surface become highly rotationally excited, whereas a very small change in the rotational state of the scattered molecules is observed for the initial parallel alignments. The latter confirms the energy transfer dependence on the stereodynamics for the present system. The results of our simulations are in overall agreement with the experimental observations regarding the shape of the angular distributions and the alignment dependence of the in-plane reflected molecules.

Using density functional theory and an exchange–correlation functional that includes the van der Waals interaction, we study the coadsorption of CO on Ru(0001) saturated with 0.5 ML of oxygen. Different coexisting CO coverages are considered that are experimentally motivated, the room temperature coverage consisting of 0.5 ML-O + 0.25 ML-CO (low coverage), the saturation coverage achieved 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). For each coverage, we study the competition between the desorption and oxidation of CO on the corresponding optimized structure by analyzing their reaction energies and minimum energy reaction paths. The desorption process is endothermic at all coverages, although the desorption energy decreases as the CO coverage increases. The process itself (and also the reverted adsorption) becomes more involved at the intermediate and high coverages because of the appearance of a physisorption well and concomitant energy barrier separating it from the chemisorbed state. Remarkably, the oxidation of CO, which is endothermic at low coverages, turns exothermic at the intermediate and high coverages. In all cases, the minimum reaction path for oxidation, which involves the chemisorbed and physisorbed CO2, is ruled by one of the large energy barriers that protect these molecular states. Altogether, the larger activation energies for oxidation as compared to those for desorption and the extreme complexity of the oxidation against the desorption paths explain that CO desorption dominates over the oxidation in experiments.

Experimental evidence exists for the enhancement of the hydrogen storage capacity of porous carbons when these materials are doped with metal nanoparticles. One of the most studied dopants is palladium. Dissociation of the hydrogen molecules and spillover of the H atoms towards the carbon substrate has been advocated as the reason for the enhancement of the storage capacity. We have investigated this mechanism by performing ab initio density functional molecular dynamics (AIMD) simulations of the deposition of molecular hydrogen on Pd6 clusters anchored on graphene vacancies. The clusters are initially near-saturated with atomic and molecular hydrogen. This condition would facilitate the occurrence of spillover, since our energy calculations based on density functional theory indicate that migration of preadsorbed H atoms towards the graphene substrate becomes exothermic on Pd clusters with high hydrogen coverages. However, AIMD simulations show that the H atoms prefer to intercalate and absorb within the Pd cluster rather than migrate to the carbon substrate. These results reveal that high activation barriers exist preventing the spillover of hydrogen from the anchored Pd clusters to the carbon substrate.

Modeling the ultrafast photoinduced dynamics and reactivity of adsorbates on metals requires including the effect of the laser-excited electrons and, in many cases, also the effect of the highly excited surface lattice. Although the recent ab initio molecular dynamics with electronic friction and thermostats, (Te,Tl)-AIMDEF [Alducin, M.; Phys. Rev. Lett. 2019, 123, 246802], enables such complex modeling, its computational cost may limit its applicability. Here, we use the new embedded atom neural network (EANN) method [Zhang, Y.; J. Phys. Chem. Lett. 2019, 10, 4962] to develop an accurate and extremely complex potential energy surface (PES) that allows us a detailed and reliable description of the photoinduced desorption of CO from the Pd(111) surface with a coverage of 0.75 monolayer. Molecular dynamics simulations performed on this EANN-PES reproduce the (Te,Tl)-AIMDEF results with a remarkable level of accuracy. This demonstrates the outstanding performance of the obtained EANN-PES that is able to reproduce available density functional theory (DFT) data for an extensive range of surface temperatures (90–1000 K); a large number of degrees of freedom, those corresponding to six CO adsorbates and 24 moving surface atoms; and the varying CO coverage caused by the abundant desorption events.

In this chapter, we will provide the theoretical foundations on which the local-density friction approximation (LDFA) is based and examples of its application to gas–surface interaction problems. With this aim, first in Sect. 28.2 we will review the derivation of the stopping power (energy lost per unit path length) for an atom or molecule traveling through a uniform electron gas in the strong coupling limit, i. e., when its velocity is lower than the Fermi velocity of the system. Real systems present electronic density nonlinearities. For this reason in Sect. 28.3, we show how this method for calculating the stopping power has been successful to reproduce and explain experimental energy-loss measurements for ions/atoms traveling through real solids and interacting with metal surfaces. The last part of Sect. 28.3 is devoted to the description of the LDFA method that accounts for the effect of energy losses in the dynamics of thermal and hyperthermal gas particles interacting with metal surfaces. Its implementation both in molecular dynamics performed in precalculated potential energy surfaces (PESs) and in ab-initio molecular dynamics is also discussed. Finally, an overview of the knowledge acquired during last years by the application of the LDFA will be presented in Sect. 28.4. In particular, we will analyze the importance of electron–hole (e–h) pair excitations in different gas–surface elementary processes that involve atoms and molecules of practical interest, such as H, N, H2, N2, and H2O. The analysis will mostly review the results obtained for the dissociative and nondissociative adsorption, for the Eley–Rideal and hot-atom recombinations and for the scattering of these gas species on different metal surfaces. As a general conclusion, it will be shown that e–h pair excitations are typically relevant for long-lasting processes. The last part of this section will review recent applications of the LDFA to model the desorption dynamics of atoms or molecules induc

Using density functional theory combined with an evolutionary algorithm for crystal structure prediction, we study the elastic and electronic properties of various multi-principal element alloys that are based on CoCrFeNi. In total, nine quinary and one senary CoCrFeNiX (X=Ti, V, Mn, MnV, Cu, Zr, Nb, Mo, Al, Al2) alloys are studied along with the base CoCrFeNi alloy. The aim of the current study is twofold. First, we test and confirm the ability of the presented methodology to predict the crystal structure of the multi-principal element alloys based on Co, Cr, Fe, and Ni elements. Second, we calculate and compare the elastic properties of the CoCrFeNiX alloys, as well as their electronic properties, in an attempt to establish possible correlations between them. Taking CoCrFeNi as the reference alloy, our first-principles calculations of various elastic moduli (bulk, Young, and shear moduli) show that only the bulk moduli of the alloys with Cu, Mo, or Nb (in this order) are expected to be larger. Furthermore, our comparative analysis of the CoCrFeNiX alloys containing partially filled 3d and 4d elements shows that the filling of the d-shell causes a general increase in all the elastic moduli. The only exception is the decreasing behavior of the bulk modulus in the case of alloys with partially filled 3d elements.

Carbon monoxide on copper surfaces continues to be a fascinating, rich microlab for many questions evolving in surface science. Recently, hot-electron mediated, femtosecond-laser pulse induced dynamics of CO molecules on Cu(100) were the focus of experiments [Inoue, Phys. Rev. Lett. 117, 186101 (2016)10.1103/PhysRevLett.117.186101] and theory [Novko, Phys. Rev. Lett. 122, 016806 (2019)10.1103/PhysRevLett.122.016806], unraveling details of the vibrational nonequilibrium dynamics on ultrashort (subpicoseconds) timescales. In the present work, full-dimensional time-resolved hot-electron driven dynamics are studied by molecular dynamics with electronic friction (MDEF). Dissipation is included by a friction term in a Langevin equation which describes the coupling of molecular degrees of freedom to electron-hole pairs in the copper surface, calculated from gradient-corrected density functional theory (DFT) via a local density friction approximation (LDFA). Relaxation due to surface phonons is included by a generalized Langevin oscillator model. The hot-electron induced excitation is described via a time-dependent electronic temperature, the latter derived from an improved two-temperature model. Our parameter-free simulations on a precomputed potential energy surface allow for excellent statistics, and the observed trends are confirmed by on-the-fly ab initio molecular dynamics with electronic friction (AIMDEF) calculations. By computing time-resolved frequency maps for selected molecular vibrations, instantaneous frequencies, probability distributions, and correlation functions, we gain microscopic insight into hot-electron driven dynamics and we can relate the time evolution of vibrational internal CO stretch-mode frequencies to measured data, notably an observed redshift. Quantitatively, the latter is found to be larger in MDEF than in experiment and possible reasons are discussed for this observation. In our model, in addition we observe the excitation and time evoluti

Ab initio dynamical simulations based on the density functional formalism have been performed for molecular hydrogen impinging on a Pd cluster anchored to a vacancy defect in graphene. Under the conditions assumed in the simulations, most H molecules rebound after colliding with the Pd cluster, but a number of molecules stay adsorbed on its surface. Depending on whether the substrate is initially at 0 or 300 K, either one-third or one-half of those adsorbed molecules later on dissociate on the cluster, leading to two chemisorbed H atoms. For both substrate temperatures, dissociation of H triggers a transition from the original octahedral structure of the anchored Pd to an incomplete pentagonal bypiramid structure, which is essentially produced by a severe elongation of the distance between two particular neighbor Pd atoms. Interestingly, no such structural changes were previously observed for Pd adsorbed on pristine graphene. Although this new result comes for a specific reaction - this one occurring, for instance, in the anode of hydrogen fuel cells - we anticipate that the observation of a structural change, which means that the cluster structure is not immune to the reaction taking place on its surface, can be relevant for many catalytic processes occurring on the surface of small metal particles.

Time-resolved vibrational spectroscopy constitutes an invaluable experimental tool for monitoring hotcarrier-induced surface reactions. However, the absence of a full understanding of the precise microscopic mechanisms causing the transient spectral changes has limited its applicability. Here we introduce a robust first-principles theoretical framework that successfully explains both the nonthermal frequency and linewidth changes of the CO internal stretch mode on Cu(100) induced by femtosecond laser pulses. Two distinct processes engender the changes: electron-hole pair excitations underlie the nonthermal frequency shifts, while electron-mediated vibrational mode coupling gives rise to linewidth changes. Furthermore, the origin and precise sequence of coupling events are finally identified.

The TiZrNbTaMo multi-principal element alloy has been postulated as a new potential material for biomedical applications, such as orthopedic implants. Besides the good biocompatibility of the constituent atoms, the TiZrNbTaMo alloy also exhibits excellent corrosion resistance and mechanical properties, according to recent experimental studies. Motivated by these experiments, here we investigate with density functional theory (DFT) the structure, as well as the elastic and electronic properties, of the equiatomic and nearly-equiatomic TiZrNbTaMo alloys. By combining evolutionary algorithms with DFT calculations of the energy, we can correctly predict the crystal structures of the two phases that are identified in experiments. The corresponding elastic properties, which are also calculated with DFT, are in good qualitative agreement with the experimental observations. The analysis of the electronic properties allows us to explain the differences in the elastic moduli between the two phases in terms of the differences in both the electron density distribution and the bonding-states occupation.

Adiabatic and nonadiabatic quasi-classical molecular dynamics simulations are performed to investigate the role of electron-hole pair excitations in hot-atom and Eley-Rideal H2 recombination mechanisms on H-covered W(100). The influence of the surface structure is analyzed by comparing with previous results for W(110). In the two surfaces, hot-atom abstraction cross sections are drastically reduced due to the efficient energy exchange with electronic excitations at low incident energies and low coverage, while the effect on Eley-Rideal reactivity is negligible. As the coverage increases, the projectile energy is more efficiently dissipated into the other adsorbates. Consequently, the effect of electronic excitations is reduced. As a result, the reactivity and final energy distributions of the formed H2 molecules are similar for both abstraction mechanisms.

We bring forth a consistent theory for the electron-mediated vibrational intermode coupling that clarifies the microscopic mechanism behind the vibrational relaxation of adsorbates on metal surfaces. Our analysis points out the inability of state-of-the-art nonadiabatic theories to quantitatively reproduce the experimental linewidth of the CO internal stretch mode on Cu(100) and it emphasizes the crucial role of the electron-mediated phonon-phonon coupling in this regard. The results demonstrate a strong electron-mediated coupling between the internal stretch and low-energy CO modes, but also a significant role of surface motion. Our nonadiabatic theory is also able to explain the temperature dependence of the internal stretch phonon linewidth, thus far considered a sign of the direct anharmonic coupling.

Through extensive direct dynamics studies of a prototypical Eley-Rideal type process, we demonstrate that fingerprints of microscopic reaction mechanisms can be identified in product distributions. Specifically, the umbrella vibration of CD formed by reacting gaseous D atoms with CD adsorbed on Cu(111) is highly excited if the reaction is initiated by a direct collision between the reactants due to the necessary inversion of the CD moiety. On the other hand, an indirect collision via a hot atom mechanism leads to much lower excitation in this product vibrational mode, and as such, an inversion is unnecessary. This fingerprint suggests an experimentally verifiable means to disentangle different mechanistic pathways.

The reaction between an impinging H atom and a Cl atom adsorbed on Au(111), which is a prototype for the Eley-Rideal mechanism, is investigated using ab initio molecular dynamics at different incidence angles. The reaction yielding gaseous HCl with large internal excitation proceeds via both direct and hot-atom mechanisms. Significant energy exchange with both surface phonons and electron-hole pairs has been observed. However, their impact on the reactivity and final state distributions was found to be limited, thanks to the large exothermicity and small barrier of the reaction.

We study surface temperature effects on the adsorption and relaxation of N atoms on Ag(111). To this aim, we perform ab initio molecular dynamics simulations with electronic friction, in which the surface is coupled to a thermostat that fixes the desired surface temperature. Simulations performed at 80 and 700 K show that the surface temperature has minor effects on magnitudes such as the initial adsorption probability, the relaxation rate of the adsorbing N, and the energy lost in electronic excitations. Slight differences are identified in the adsorption paths with the appearance of subsurface absorption events at 700 K that are not observed at 80 K. Furthermore, we perform additional simulations without a thermostat in order to examine the validity of commonly used ab initio molecular dynamics simulations in which no heat dissipation from the simulation cell is allowed. Our results show that such a methodology may not suffice to simulate the low-temperature regime since the surface becomes unphysically heated within a few picoseconds upon adsorption of the N atom. However, neither in this unfavorable case are the magnitudes defining the dynamics of the adsorbates at the same time scale significantly modified from those obtained at constant surface temperature.

Electronic stopping of slow protons in ZnO, VO2 (metal and semiconductor phases), HfO2, and Ta2O5 was investigated experimentally. As a comparison of the resulting stopping cross sections (SCS) to data for Al2O3 and SiO2 reveals, electronic stopping of slow protons does not correlate with electronic properties of the specific material such as band gap energies. Instead, the oxygen 2p states are decisive, as corroborated by density functional theory calculations of the electronic densities of states. Hence, at low ion velocities the SCS of an oxide primarily scales with its oxygen density.

In scattering of H2 from Cu(111), vibrational excitation has so far defied an accurate theoretical description. To expose the causes of the large discrepancies with experiment, we investigate how the feature due to vibrational excitation (the “gain peak”) in the simulated time-of-flight spectrum of (v = 1, j = 3) H2 scattering from Cu(111) depends on the surface temperature (Ts) and the possibility of energy exchange with surface phonons and electron–hole pairs (ehp’s). Quasi-classical dynamics calculations are performed on the basis of accurate semiempirical density functionals for the interaction with H2 + Cu(111). The methods used include the quasi-classical trajectory method within the Born–Oppenheimer static surface model, the generalized Langevin oscillator (GLO) method incorporating energy transfer to surface phonons, the GLO + friction (GLO+F) method also incorporating energy exchange with ehp’s, and ab initio molecular dynamics with electronic friction (AIMDEF). Of the quasi-classical methods tested, comparison with AIMDEF suggests that the GLO+F method is accurate enough to describe vibrational excitation as measured in the experiments. The GLO+F calculations also suggest that the promoting effect of raising Ts on the measured vibrational excitation is due to an electronically nonadiabatic mechanism. However, by itself, enabling energy exchange with the surface by modeling surface phonons and ehp’s leads to reduced vibrational excitation, further decreasing the agreement with experiment. The simulated gain peak is quite sensitive to energy shifts in calculated vibrational excitation probabilities and to shifts in a specific experimental parameter (the chopper opening time). While the GLO+F calculations allow important qualitative conclusions, comparison to quantum dynamics results suggests that, with the quasi-classical way of describing nuclear motion and the present box quantization method for assigning the final vibrational state, the gain peak is not y

Using adiabatic and nonadiabatic quasiclassical molecular dynamics simulations within the single adsorbate limit, we study the dependence on the incidence angle of the Eley-Rideal (ER) recombination of H and N on the (100) and (110) tungsten surfaces. In the nonadiabatic simulations, effects due to electron-hole pair excitations and the energy exchange between the recombination partners and the surface lattice are included in the dynamics by combining the local density friction approximation and the generalized Langevin oscillator model. Our adiabatic and nonadiabatic results, which are qualitatively similar, show that the incidence angle affects much more N recombination than H recombination due mainly to the higher corrugation of the potential energy surface for the former case. In particular, for N abstraction from W(110), we find a very efficient ER recombination pathway at grazing incidence angles. This new mechanism significantly influences the energy partition among the different degrees of freedom of the outgoing molecules. We also find that the energy losses only depend slightly on the incidence geometry and that, as in normal incidence, they are dominated by phonons in the case of N recombination and by electron-hole pair excitations in the case of H recombination.

Great success has been achieved in the modeling of gas-surface elementary processes by the use of the Born-Oppenheimer approximation. However, in metal surfaces low energy electronic excitations are generated even by thermal and hyperthermal molecules due to the absence of band gaps in the electronic structure. This shows the importance of performing dynamical simulations that incorporate non-adiabatic effects to analyze in which way they affect most common gas-surface reactions. Here we review recent theoretical developments in this problem and their application to the study of the effect of electronic excitations in the adsorption and relaxation of atoms and molecules in metal surfaces, in scattering processes, and also in recombinative processes between impinging atoms and adsorbates at the surface. All these studies serve us to establish what properties of the gas-surface interaction favor the excitation of low-energy electron-hole pairs. A general observation is that the nature of these excitations usually requires long lasting interactions at the surface in order to observe deviations from the adiabatic behaviour. We also provide the basis of the local density friction approximation (LDFA) that have been used in all these studies, and show how it has been employed to perform ab initio molecular dynamics with electronic friction (AIMDEF). As a final remark, we will shortly review on recent applications of the LDFA to successfully simulate desorption processes induced by intense femtosecond laser pulses.