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...