Inelastic scattering of H atoms from surfaces

We have developed an instrument that uses photolysis of hydrogen halides to produce nearly monoenergetic hydrogen atom beams and Rydberg atom tagging to obtain accurate angle-resolved time-of-flight distributions of atoms scattered from surfaces. The surfaces are prepared under strict ultrahigh vacuum conditions. Data from these experiments can provide excellent benchmarks for theory, from which it is possible to obtain an atomic scale understanding of the underlying dynamical processes governing H atom adsorption. In this way, the mechanism of adsorption on metals is revealed, showing a penetration–resurfacing mechanism that relies on electronic excitation of the metal by the H atom to succeed. Contrasting this, when H atoms collide at graphene surfaces, the dynamics of bond formation involving at least four carbon atoms govern adsorption. Future perspectives of H atom scattering from surfaces are also outlined

Effective medium theory for bcc metals: Electronically non-adiabatic H atom scattering in full dimensions

In summary, we have extended the EMT formalism derived for fcc metals22 to the bcc case. We then fit the newly derived formulae to DFT data for H interacting with W and Mo, which led to full dimensional PESs and electron densities. We employed the PESs and the electron densities to carry out electronically non-adiabatic MD simulations of H atom scatter- ing, following previous work that used the LDFA approximation with a Langevin propagator. Specifically, we predict energy loss distributions for H scattering from (111) and (110) facets of these two metals at 2.76 eV incidence energy. Although no experiments are currently available for bcc metals, our results are similar to what has been seen for H scattering from fcc metals. This suggests that the current results are likely to be a reliable prediction of experiment. We find only subtle differ- ences in the energy loss distributions arising from the scatter- ing of H atom with these two metals; however, scattering from the (111) and (110) facets are distinctly different. Remarkably, on the (110) facet, we predict a clearly resolvable energy loss peak that arises from sub-surface scattering. The calculations Fig. 9 Distribution of specular scattering events as a function of the energy loss and the depth of penetration of H atom scattered from (a) Mo(110), (b) Mo(111), (c) W(110), and (d) W(111). The surface temperature is 70 K. The other conditions are the same as in Fig. 8. The signal above the black, dashed line indicate from which layer the projectiles repelled. The labels top, hcp and fcc refer to the high-symmetry sites of the (111) facet and are shown in Fig. 1(b). The bin sizes are 0.027 eV and 0.063 Å. Table 3 Sticking coefficient S0 computed from the same set of trajec- tories that were used for the calculation of the specular energy loss distributions shown in Fig. 8 System 300 K 70 K H/Mo(110) 0.44 0.44 H/Mo(111) 0.40 0.41 H/W(110) 0.42 0.41 H/W(111) 0.40 0.40 Paper PCCPOpen Access Article. Published on 04 April 2022. Downloaded on 6/8/2022 3:06:58 PM.This article is licensed under aCreative Commons Attribution 3.0 Unported Licence.View Article Online 8746 | Phys. Chem. Chem. Phys., 2022, 24, 8738–8748 This journal is © the Owner Societies 2022predict that the subsurface scattering is most easily seen for H scattering from W(110) at reduced surface temperatures

Multibounce and subsurface scattering of H atoms colliding with a van der Waals solid

We report the results of inelastic differential scattering experiments and full-dimensional molecular dynamics trajectory simulations for 2.76 eV H atoms colliding at a surface of solid xenon. The interaction potential is based on an effective medium theory (EMT) fit to density functional theory (DFT) energies. The translational energy-loss distributions derived from experiment and theory are in excellent agreement. By analyzing trajectories, we find that only a minority of the scattering results from simple single-bounce dynamics. The majority comes from multibounce collisions including subsurface scattering where the H atoms penetrate below the first layer of Xe atoms and subsequently re-emerge to the gas phase. This behavior leads to observable energy-losses as large as 0.5 eV, much larger than a prediction of the binary collision model (0.082 eV), which is often used to estimate the highest possible energy-loss in direct inelastic surface scattering. The sticking probability computed with the EMT-PES (0.15) is dramatically reduced (5 × 10–6) if we employ a full-dimensional potential energy surface (PES) based on Lennard-Jones (LJ) pairwise interactions. Although the LJ-PES accurately describes the interactions near the H–Xe and Xe–Xe energy minima, it drastically overestimates the effective size of the Xe atom seen by the colliding H atom at incidence energies above about 0.1 eV

Toward detection of electron-hole pair excitation in H-atom collisions with Au(111): Adiabatic molecular dynamics with a semi-empirical full-dimensional potential energy surface.

We report an analytic potential energy surface (PES) based on several hundred DFT energies for H interacting with a Au(111) surface. Effective medium theory is used to fit the DFT data, which were obtained for the Au atoms held at their equilibrium positions. This procedure also provides an adequate treatment of the PES for displacements of Au atoms that occur during scattering of H atoms. The fitted PES is compared to DFT energies obtained from ab initio molecular dynamics trajectories. We present molecular dynamics simulations of energy and angle resolved scattering probabilities at five incidence angles at an incidence energy, Ei = 5 eV, and at a surface temperature, TS = 10 K. Simple single bounce trajectories are important at all incidence conditions explored here. Double bounce events also make up a significant fraction of the scattering. A qualitative analysis of the double-bounce events reveals that most occur as collisions of an H-atom with two neighboring surface gold atoms. The energy losses observed are consistent with a simple binary collision model, transferring typically less than 150 meV to the solid per bounce

An axis-specific rotational rainbow in the direct scatter of formaldehyde from Au(111) and its influence on trapping probability.

The conversion of translational to rotational motion often plays a major role in the trapping of small molecules at surfaces, a crucial first step for a wide variety chemical processes that occur at gas-surface interfaces. However, to date most quantum-state resolved surface scattering experiments have been performed on diatomic molecules, and little detailed information is available about how the structure of nonlinear polyatomic molecules influences the mechanisms for energy exchange with surfaces. In the current work, we employ a new rotationally resolved 1 + 1' resonance-enhanced multiphoton ionization (REMPI) scheme to measure the rotational distribution in formaldehyde molecules directly scattered from the Au(111) surface at incidence kinetic energies in the range 0.3-1.2 eV. The results indicate a pronounced propensity to excite a-axis rotation (twirling) rather than b- or c-axis rotation (tumbling or cartwheeling), and are consistent with a rotational rainbow scattering model. Classical trajectory calculations suggest that the effect arises-to zeroth order-from the three-dimensional shape of the molecule (steric effects). Analysis suggests that the high degree of rotational excitation has a substantial influence on the trapping probability of formaldehyde at incidence translational energies above 0.5 eV

Condensed-phase isomerization through tunnelling gateways

Quantum mechanical tunnelling describes transmission of matter waves through a barrier with height larger than the energy of the wave. Tunnelling becomes important when the de Broglie wavelength of the particle exceeds the barrier thickness; because wavelength increases with decreasing mass, lighter particles tunnel more efficiently than heavier ones. However, there exist examples in condensed-phase chemistry where increasing mass leads to increased tunnelling rates. In contrast to the textbook approach, which considers transitions between continuum states, condensed-phase reactions involve transitions between bound states of reactants and products. Here this conceptual distinction is highlighted by experimental measurements of isotopologue-specific tunnelling rates for CO rotational isomerization at an NaCl surface, showing nonmonotonic mass dependence. A quantum rate theory of isomerization is developed wherein transitions between sub-barrier reactant and product states occur through interaction with the environment. Tunnelling is fastest for specific pairs of states (gateways), the quantum mechanical details of which lead to enhanced cross-barrier coupling; the energies of these gateways arise nonsystematically, giving an erratic mass dependence. Gateways also accelerate ground-state isomerization, acting as leaky holes through the reaction barrier. This simple model provides a way to account for tunnelling in condensed-phase chemistry, and indicates that heavy-atom tunnelling may be more important than typically assumed

Random force in molecular dynamics with electronic friction

Originally conceived to describe thermal diffusion, the Langevin equation includes both a frictional drag and a random force, the latter representing thermal fluctuations first seen as Brownian motion. The random force is crucial for the diffusion problem as it explains why friction does not simply bring the system to a standstill. When using the Langevin equation to describe ballistic motion, the importance of the random force is less obvious and it is often omitted, for example, in theoretical treatments of hot ions and atoms interacting with metals. Here, friction results from electronic nonadiabaticity (electronic friction), and the random force arises from thermal electron–hole pairs. We show the consequences of omitting the random force in the dynamics of H-atom scattering from metals. We compare molecular dynamics simulations based on the Langevin equation to experimentally derived energy loss distributions. Despite the fact that the incidence energy is much larger than the thermal energy and the scattering time is only about 25 fs, the energy loss distribution fails to reproduce the experiment if the random force is neglected. Neglecting the random force is an even more severe approximation than freezing the positions of the metal atoms or modelling the lattice vibrations as a generalized Langevin oscillator. This behavior can be understood by considering analytic solutions to the Ornstein–Uhlenbeck process, where a ballistic particle experiencing friction decelerates under the influence of thermal fluctuations

Multiquantum vibrational excitation of NO scattered from Au(111): quantitative comparison of benchmark data to Ab initio theories of nonadiabatic molecule-surface interactions.

Measurements of absolute probabilities are reported for the vibrational excitation of NO(v=0→1,2) molecules scattered from a Au(111) surface. These measurements were quantitatively compared to calculations based on ab initio theoretical approaches to electronically nonadiabatic molecule–surface interactions. Good agreement was found between theory and experiment (see picture; Ts=surface temperature, P=excitation probability, and E=incidence energy of translation)

Steric Hindrance of NH3 Diffusion on Pt(111) by Co-Adsorbed O-Atoms

A detailed velocity-resolved kinetics study of NH3 thermal desorption rates from p(2 × 2) O/Pt(111) is presented. We find a large reduction in the NH3 desorption rate due to adsorption of O-atoms on Pt(111). A physical model describing the interactions between adsorbed NH3 and O-atoms explains these observations. By fitting the model to the derived desorption rate constants, we find an NH3 stabilization on p(2 × 2) O/Pt(111) of 0.147–0.014+0.023 eV compared to Pt(111) and a rotational barrier of 0.084–0.022+0.049 eV, which is not present on Pt(111). The model also quantitatively predicts the steric hindrance of NH3 diffusion on Pt(111) due to co-adsorbed O-atoms. The derived diffusion barrier of NH3 on p(2 × 2) O/Pt(111) is 1.10–0.13+0.22 eV, which is 0.39–0.14+0.22 eV higher than that on pristine Pt(111). We find that Perdew Burke Ernzerhof (PBE) and revised Perdew Burke Ernzerhof (RPBE) exchange–correlation functionals are unable to reproduce the experimentally observed NH3–O adsorbate–adsorbate interactions and NH3 binding energies at Pt(111) and p(2 × 2) O/Pt(111), which indicates the importance of dispersion interactions for both systems