Hydrogen collisions with transition metal surfaces: Universal electronically nonadiabatic adsorption

Abstract

Inelastic scattering of H and D atoms from the (111) surfaces of six fcc transition metals (Au, Pt, Ag, Pd, Cu, and Ni) was investigated, and in each case, excitation of electron-hole pairs dominates the inelasticity. The results are very similar for all six metals. Differences in the average kinetic energy losses between metals can mainly be attributed to different efficiencies in the coupling to phonons due to the different masses of the metal atoms. The experimental observations can be reproduced by molecular dynamics simulations based on full-dimensional potential energy surfaces and including electronic excitations by using electronic friction in the local density friction approximation. The determining factors for the energy loss are the electron density at the surface, which is similar for all six metals, and the mass ratio between the impinging atoms and the surface atoms. Details of the electronic structure of the metal do not play a significant role. The experimentally validated simulations are used to explore sticking over a wide range of incidence conditions. We find that the sticking probability increases for H and D collisions near normal incidence-consistent with a previously reported penetration-resurfacing mechanism. The sticking probability for H or D on any of these metals may be represented as a simple function of the incidence energy, E, metal atom mass, M, and incidence angle, 𝜗. S=(S+a⋅E+b⋅M)*(1-h(𝜗-c)(1-cos(𝜗-c))), where h is the Heaviside step function and for H, S = 1.081, a = -0.125 eV, b=-8.40⋅10 u, c = 28.88°, d = 1.166 eV, and e = 0.442 eV; whereas for D, S = 1.120, a = -0.124 eV, b=-1.20⋅10 u, c = 28.62°, d = 1.196 eV, and e = 0.474 eV.

Inelastic H and D atom scattering from Au(111) as benchmark for theory

“…Theoretical calculations predict an average number of 2-3 depending on the incidence energy. 19 H atoms that underwent a single collision are preferentially observed close to the specular angle, while H atoms that underwent several collisions are expected to follow a cos θ angular distribution peaking at the surface normal. This explains the observed scattering angle dependency of the energy loss: At specular angle, single bounce events are observed with higher probability, while at surface normal, multibounce events dominate.…”

“…The large energy loss is due to an efficient energy transfer to electron-hole-pair excitations. [17][18][19] In the following, three aspects of the scattering experiments are discussed in detail: First, the influence of the incidence parameters of the H atom beam, second the influence of the azimuthal orientation of the crystal, and third the isotope effect.…”

“…Going to lower incidence energies is, in principle, possible, but the large sticking probabilities for H on Au(111) for incidence energies <0.9 eV prohibit scattering experiments. 19 The size of the crystal and the geometry of the sample mount limit the polar incidence angle to a maximum of 75 ○ . Instead of measuring the complete angle-resolved translational energy distribution, we measured energy integrated angular distributions and energy loss spectra at selected scattering angles, making direct comparisons between results for different incidence conditions straight forward.…”

“…For a scattering angle of 0 ○ , the energy loss becomes independent of the incidence angle. The reason for larger error bars for lower incidence energies is the sticking probability of H to Au(111): It increases from 48% for an incidence energy of 3.33 eV to 79% for 0.99 eV, 19 leading to a lower scattering signal for smaller incidence energies. Figure 5 shows the dependence of the average energy loss on the incidence energy for one incidence angle and two scattering angles.…”

“…16 Recently, we performed inelastic scattering experiments of H and D atoms from metal surfaces. [17][18][19] A comparison between an insulator and a metal surface showed that a large amount of translational energy is transferred to electronic excitations of the metal surface in the scattering event, explaining the high adsorption probabilities. Kandratsenka et al developed a theoretical model based on molecular dynamics (MD) simulations on a global full-dimensional potential energy surface (PES) to simulate the inelastic scattering experiments.…”