Ab initiomolecular dynamics with simultaneous electron and phonon excitations: Application to the relaxation of hot atoms and molecules on metal surfaces

Abstract

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...

Role of Tensorial Electronic Friction in Energy Transfer at Metal Surfaces

“…The applicability of LDFA for molecular adsorbates has been a topic of recent discussion [22,27,40]. Table I shows results for an embedding density based on the density of the clean surface (Independent Atom Approximation, LDFA-IAA) and an embedding density as given by the atoms-in-molecules scheme (LDFA-AIM) [40,42]. Analogously to the analysis of hydrogen, we can calculate atom-wise isotropic friction coefficients from the TDPT tensor by averaging over diagonal elements and discarding off-diagonals.…”

Communication: Hot-atom abstraction dynamics of hydrogen from tungsten surfaces: The role of surface structure

“…For hydrogen atoms and molecules interacting with metallic surfaces at low coverage, for which collisions with pre-adsorbed species are unlikely, the main energy dissipation channel in the picosecond time scale is the excitation of low lying e-h pairs. [45][46][47][48][49][50][51][52] As such an effect depends on the surface electronic structure, its sensitivity to the crystal face is a relevant issue that we address in the present study. To this end, we compare, in the following, the dynamics of H 2 formation by abstraction of pre-adsorbed H atoms upon H atom scattering at finite coverage for the W(100) and W(110) surfaces.…”

“…In the latter, electronic nonadiabaticity is introduced through a dissipative force in the classical equations of motion for the hydrogen atoms. This approach has already been used to investigate non-adiabatic effects in gas-surface elementary processes 46,[48][49][50][51][61][62][63] and allows a good compromise between accuracy and simplicity. 51,64 The friction force, acting independently on each H atom, is approximated as the one corresponding to a homogeneous free electron gas with electronic density equal to that of the bare surface at the atom position.…”

“…51,64 The friction force, acting independently on each H atom, is approximated as the one corresponding to a homogeneous free electron gas with electronic density equal to that of the bare surface at the atom position. This approximation has been recently used to rationalize the adsorption and relaxation of H, N, and N 2 on bare metallic surfaces, 46,48,50 as well as for recombinative H 2 desorption induced by fs-laser pulses. 65,66 Dissipation to surface phonons is neglected here as dissipation to electrons has been recently predicted to largely dominate the relaxation of hydrogen on metals in the sub-picosecond time scale.…”

“…65,66 Dissipation to surface phonons is neglected here as dissipation to electrons has been recently predicted to largely dominate the relaxation of hydrogen on metals in the sub-picosecond time scale. [45][46][47][48] As a result of H-atom scattering, recombination may occur via three different processes: ER abstraction is assumed when the formed molecule moves definitively toward the vacuum after the first rebound of the projectile. 37 Otherwise, abstraction is considered as primary HA process.…”