We quantitatively evaluate the contribution of electron-hole pair excitations to the reactive dynamics of H 2 on Cu(110) and N 2 on W(110), including the six dimensionality of the process in the entire calculation. The interaction energy between molecule and surface is represented by an ab initio six-dimensional potential energy surface. Electron friction coefficients are calculated with density functional theory in a local density approximation. Contrary to previous claims, only minor differences between the adiabatic and nonadiabatic results for dissociative adsorption are found. Our calculations demonstrate the validity of the adiabatic approximation to analyze adsorption dynamics in these two representative systems. DOI: 10.1103/PhysRevLett.100.116102 PACS numbers: 82.65.+r, 34.35.+a, 68.49.Df, 82.20.Kh The adiabatic Born-Oppenheimer approximation is ubiquitous in the theoretical study of elementary reactive processes at surfaces. Still, there is ample experimental evidence of electronic excitations associated to gas or surface reactions, that can potentially break down the applicability of the adiabatic approach. Electron-hole pairs appear, for instance, in the detection of chemicurrents during the chemisorption of gas-phase species on thin metal films [1,2], as well as in the measurement of electron emission following the scattering of molecules in highlyexcited vibrational states on metal surfaces [3,4]. Although the existence of energy dissipation through electron-hole (e-h) pair excitations is widely accepted, there is not a definite quantitative answer on the role of electronic excitations in the adsorption and reaction rates of diatomic molecules at metal surfaces [5].Laursen et al. [6] predicted that e-h pair excitations should alter substantially the adsorption dynamics of H 2 in Cu (110). Contravening this prediction, full sixdimensional (6D) adiabatic calculations of the dynamics based on an ab initio potential energy surface have been shown to provide a good description of the experimental results on this system [7]. Measurements of the dissociative adsorption and diffractive scattering of H 2 on Pt(111) are reasonably well described within the adiabatic approximation as well [8]. For heavier molecules, it was claimed that strong energy dissipation effects due to the excitation of e-h pairs are responsible for the strong disagreement between (adiabatic) theoretical and experimental sticking coefficients for N 2 on Ru(0001) [9,10]. Díaz et al. [11] showed afterwards that when the full dimensionality of the process is taken into account, adiabatic calculations are much closer to experiments. This result suggests that nonadiabatic effects might be smaller than previously predicted. Still, the lack of a theoretical calculation, based on state-of-the-art interaction potentials, that explicitly includes the e-h pair excitation channel and the full dimensionality of the process keeps this controversy open.In this Letter, we evaluate the contribution of e-h pair excitations to the dissociative adsorp...

We study the dynamics of transient hot H atoms on Pd(100) that originated from dissociative adsorption of H 2 . The methodology developed here, denoted AIMDEF, consists of ab initio molecular dynamics simulations that include a friction force to account for the energy transfer to the electronic system. We find that the excitation of electron-hole pairs is the main channel for energy dissipation, which happens at a rate that is five times faster than energy transfer into Pd lattice motion. Our results show that electronic excitations may constitute the dominant dissipation channel in the relaxation of hot atoms on surfaces. DOI: 10.1103/PhysRevLett.112.103203 PACS numbers: 68.43.-h, 34.35.+a, 34.50.Bw, 82.20.Gk Electron-hole (e-h) pair excitations are an unquestioned efficient energy drain in the interaction of fast atoms with solids and surfaces [1][2][3][4]. In contrast, the relevance of this dissipation channel in gas-surface interactions that involve energies up to a few eV is not so clear. It depends not only on the specific system, but also on the elementary process considered, as shown by different studies on scattering [5][6][7][8][9][10][11][12] and adsorption [6,[13][14][15][16][17][18][19][20][21][22][23][24][25][26] of atoms and molecules on surfaces. Low-energy e-h pair excitations have been detected as chemicurrents on Schottky diode devices during the chemisorption of atomic and molecular species on metals [13][14][15]. The correlation found between chemicurrent intensities and adsorption energies is a strong indication that a large fraction of the energy dissipated in both the dissociative and nondissociative adsorption processes is used to excite e-h pairs. However, this strongly contrasts with examples showing that the dissociative adsorption is reasonably well described within the electronically adiabatic approach, which neglects the coupling between electronic excitations and the nuclear motion [6,[18][19][20]23,25], and that the effect of electronic energy dissipation seems negligible [21,24,26]. Therefore, a question that is raised here is at what stage of the dissociative adsorption process e-h pair excitations do become relevant.In a typical adsorption event, the incoming gas species gain additional kinetic energy when entering the attractive adsorption well. In the particular case of dissociative adsorption, this energy gain can lead to the formation of so-called "hot" atoms or fragments, with energies much larger than the corresponding thermal energies of the substrate atoms. The formed hot species will then propagate along the surface until they dissipate the excess kinetic energy and finally accommodate at a stable adsorption position. This stage of the dissociative adsorption process, where the relevance of e-h pair excitations has been traditionally neglected, is the focus of the present work.In this Letter we show that while e-h pair excitation may not be relevant on the molecule-bond-breaking time scale, it is an efficient dissipative channel in the subsequent relaxation of the...

Juaristi et al. Reply: In the preceding Comment [1] on our recent Letter [2], Luntz et al. claim that their results show that the local density approximation for electronic friction (LDAF) is in general not reliable for estimating the electronic friction for molecules on metal surfaces. This is an unfounded statement based on very limited information.Our LDAF calculation for the friction coefficient involves at most two further approximations as compared to those involved in the model used by Luntz et al.. (i) First, a local approximation for the electronic density is used. This is acknowledged to be a reasonable approximation. The LDAF was shown to be appropriate for calculations of the friction coefficient for atoms in metal surfaces, incidentally by some of the authors of the comment [3]. (ii) An additional approximation is that two-center effects are disregarded. Two-center effects were shown not to change the order of magnitude in the case of H 2 [4], but no studies in other systems have been performed to our knowledge.

The relaxation dynamics of hot H, N, and N 2 on Pd(100), Ag(111), and Fe(110), respectively, is studied by means of ab initio molecular dynamics with electronic friction. This method is adapted here to account for the electron density changes caused by lattice vibrations, thus treating on an equal footing both electron-hole (e-h) pair and phonon excitations. We find that even if the latter increasingly dominate the heavier is the hot species, the contribution of e-h pairs is by no means negligible in these cases because it gains relevance at the last stage of the relaxation process. The quantitative details of energy dissipation depend on the interplay of the potential energy surface, electronic structure, and kinetic factors. DOI: 10.1103/PhysRevB.92.201411 PACS number(s): 82.65.+r, 34.35.+a, 34.50.Bw, 68.43.−h In dynamic gas-surface environments, where gas-phase atomic and molecular species impinge on the surface at energies of the order of up to a few eV, energy dissipation occurs by the excitation of electron-hole (e-h) pairs and the excitation of lattice vibrations, i.e., phonons [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In the adsorption processes of atomic and molecular species, dissociative as well as nondissociative, the species trapped by the surface gradually lose their energy until they become thermalized on the surface. The competition between the e-h pairs and phonon channels governs the relaxation dynamics of the transient hot species, and thus it plays a decisive role in the system reactivity properties. The reason is that it rules the traveled length and relaxation time of a hot atom or molecule on the surface and, consequently, the probability to undergo a recombination reaction with another adsorbate [18][19][20][21][22][23].Recent ab initio molecular dynamics (AIMD) simulations with electronic friction (AIMDEF) have shown that e-h pair excitations are the dominant relaxation mechanism for hot H atoms on Pd(100) that originate from the dissociative adsorption of H 2 [16]. More particularly, this channel dissipates energy at a five times faster rate than the phonon channel [10]. The two main reasons behind this behavior are the long H-Pd interaction time, of hundreds of fs, and the low adsorbate-tosurface atom mass ratio, γ = m H /m Pd = 0.0094. The case of H on Pd(100) represents a limiting case. For heavier adsorbates, the relative weight of e-h pairs and phonons in the energy loss is expected to vary. The energy transfer to the substrate will be determined not only by kinetic factors, such as the value of γ and the incidence conditions, but also by the topography of the multidimensional potential energy surface (PES) and the electronic structure details of the configurations probed along the relaxation trajectory.In this Rapid Communication, we investigate the relaxation dynamics of hot species in three adsorption scenarios that are representative of different energy loss regimes. We have chosen atomic N on Ag(111) (γ = 0.13), N 2 on Fe(110) (γ = 0.5), and the aforemen...

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Ab initio molecular dynamics with electronic friction (AIMDEF) is a valuable methodology to study the interaction of atomic particles with metal surfaces. This method, in which the effect of low-energy electron-hole (e-h) pair excitations is treated within the local density friction approximation (LDFA) [Juaristi et al., Phys. Rev. Lett. 100, 116102 (2008)], can provide an accurate description of both e-h pair and phonon excitations. In practice, its applicability becomes a complicated task in those situations of substantial surface atoms displacements because the LDFA requires the knowledge at each integration step of the bare surface electron density. In this work, we propose three different methods of calculating on-the-fly the electron density of the distorted surface and we discuss their suitability under typical surface distortions. The investigated methods are used in AIMDEF simulations for three illustrative adsorption cases, namely, dissociated H 2 on Pd(100), N on Ag(111), and N 2 on Fe(110). Our AIMDEF calculations performed with the three approaches highlight the importance of going beyond the frozen surface density to accurately describe the energy released into e-h pair excitations in case of large surface atom displacements.

The influence of electron-hole pairs in dissociative chemisorption of a polyatomic molecule (water) on metal surfaces is assessed for the first time using a friction approach. The atomic local density dependent friction coefficients computed based on a free electron gas embedding model are employed in classical molecular dynamics simulations of the water dissociation dynamics on rigid Ni(111) using a recently developed nine dimensional interaction potential energy surface for the system. The results indicate that nonadiabatic effects are relatively small and they do not qualitatively alter the mode specificity in the dissociation.

The six-dimensional potential energy surface for the dissociation of N 2 molecules on the W͑110͒ surface has been determined by density functional calculations and interpolated using the corrugation reducing procedure. Examination of the resulting six-dimensional potential energy surface shows that nonactivated paths are available for dissociation. In spite of this, the dissociation probability goes to a very small value when the impact energy goes to zero and increases with increasing energy, a behavior usually associated with activated systems. Statistics on the dynamics indicate that this unconventional result is a consequence of the characteristics of the potential energy surface at long distances. Furthermore, two distinct channels are identified in the dissociation process, namely, a direct one and an indirect one. The former is responsible for dissociation at high energies. The latter, which includes long-lasting dynamic trapping in the vicinity of a potential well above the W top position, is the leading mechanism at low and intermediate energies.