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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.

Grazing-incidence fast-projectile diffraction has been proposed both as a complement and an alternative to thermal-energy projectile scattering, which explains the interest that this technique has received in recent years, especially in the case of atomic projectiles. On the other hand, despite the richer physics involved, molecular projectiles have received much less attention. In this work, we present a theoretical study of grazing-incidence fast-molecule diffraction of H2 from KCl(001) using a sixdimensional density functional theory based potential energy surface and a time-dependent wavepacket propagation method. The analysis of the computed diffraction patterns as a function of the molecular alignment, and their comparison with the available experimental data, where the initial distribution of rotational states in the molecule is not known, reveals a puzzling stereodynamics effect of the diffracted projectiles: diffracted molecules aligned perpendicular, or quasi perpendicular, to the surface reproduce rather well the experimental diffraction pattern, whereas those molecules aligned parallel to or tilted with respect to the surface do not behave as in the experiments. These results call for more detailed investigations of the molecular beam generation process., MICINN, Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

Motivated by recent experimental and theoretical results, we have studied the diffraction of atoms (D, 3He, 4He) from KCl(001). To perform this study, we have computed continuos potential energy surfaces (PESs) using density functional theory to obtain total interaction energies, with and without taking into account van der Waals forces, and the corrugation reduction procedure. Subsequently, we have performed quantum dynamics simulations using the multi-configuration time-dependent Hartree method. Our simulated spectra compare rather well with those recorded experimentally, specially well for 3He. The agreement is, in general, better for incidence along the [100] direction. In the case of He projectiles, the inclusion of vdW forces does not systematically improve agreement with the experiment. Finally, in agreement with similar calculations for other systems, we have found that the diffraction spectra are quite sensitive to the subtle characteristics of PES, whereas phonons and electronic excitations seem to play a minor role., MINECO, Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

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.

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.

Grazing-incidence fast-projectile diffraction has been proposed both as a complement and an alternative to thermal-energy projectile scattering, which explains the interest that this technique has received in recent years, especially in the case of atomic projectiles. On the other hand, despite the richer physics involved, molecular projectiles have received much less attention. In this work, we present a theoretical study of grazing-incidence fast-molecule diffraction of H2 from KCl(001) using a six-dimensional density functional theory based potential energy surface and a time-dependent wavepacket propagation method. The analysis of the computed diffraction patterns as a function of the molecular alignment, and their comparison with the available experimental data, where the initial distribution of rotational states in the molecule is not known, reveals a puzzling stereodynamics effect of the diffracted projectiles: diffracted molecules aligned perpendicular, or quasi perpendicular, to the surface reproduce rather well the experimental diffraction pattern, whereas those molecules aligned parallel to or tilted with respect to the surface do not behave as in the experiments. These results call for more detailed investigations of the molecular beam generation process.

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.

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.

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.

We have studied survival and rotational excitation probabilities of H 2(v i 1, J i 1) and D 2(v i 1, J i 2) upon scattering from Cu(111) using six-dimensional (6D) adiabatic (quantum and quasi-classical) and non-adiabatic (quasi-classical) dynamics. Non-adiabatic dynamics, based on a friction model, has been used to analyze the role of electron-hole pair excitations. Comparison between adiabatic and non-adiabatic calculations reveals a smaller influence of non-adiabatic effects on the energy dependence of the vibrational deexcitation mechanism than previously suggested by low-dimensional dynamics calculations. Specifically, we show that 6D adiabatic dynamics can account for the increase of vibrational deexcitation as a function of the incidence energy, as well as for the isotope effect observed experimentally in the energy dependence for H 2(D 2)/Cu(100). Furthermore, a detailed analysis, based on classical trajectories, reveals that in trajectories leading to vibrational deexcitation, the minimum classical turning point is close to the top site, reflecting the multidimensionally of this mechanism. On this site, the reaction path curvature favors vibrational inelastic scattering. Finally, we show that the probability for a molecule to get close to the top site is higher for H 2 than for D 2, which explains the isotope effect found experimentally. © 2012 American Institute of Physics.