Workspace Guardian: Investigating Awareness of Personal Workspace Between Co-Located Augmented Reality Users.

As augmented reality (AR) systems proliferate and the technology gets smaller and less intrusive, we imagine a future where many AR users will interact in the same physical locations (e.g., in shared work places and public spaces).

Accurate Reaction Probabilities for Translational Energies on Both Sides of the Barrier of Dissociative Chemisorption on Metal Surfaces.

Molecular dynamics simulations are essential for a better understanding of dissociative chemisorption on metal surfaces, which is often the rate-controlling step in heterogeneous and plasma catalysis. The workhorse quasi-classical trajectory approach ubiquitous in molecular dynamics is able to accurately predict reactivity only for high translational and low vibrational energies. In contrast, catalytically relevant conditions generally involve low translational and elevated vibrational energies.

An examination of phonon-inelastic molecule-metal scattering using reduced density matrix and stochastic wave packet methods.

We explore the application of reduced density matrix-based approaches to molecules interacting with the lattice vibrations of metals, an interaction responsible for the temperature dependence of many of the fundamental steps of catalysis. We avoid the use of simple models for the bath and instead use density functional theory to compute all molecule-phonon interactions and the properties of the lattice phonons, for methane scattering from Ir(111). We find that while the large metal mass leads to long bath correlation times, these are not significantly longer than the time over which the reduced density matrix changes due to interactions with the bath.

The trapping of methane on Ir(111): A first-principles quantum study.

We implement a fully quantum mechanical study of methane trapping on Ir(111), where the phonons, the molecule-surface interaction, and the molecule-phonon coupling are all computed from first-principles. We find that both the surface corrugation and the phonon coupling vary strongly with molecular orientation and that there is a "chemical" aspect to this due to the catalytic nature of the metal. For example, molecules with reactive orientations can approach close to surface sites with low barriers to dissociation.

Direct and trapping-mediated pathways to dissociative chemisorption: CH dissociation on Ir(111) with step defects.

The indirect chemisorption of methane on a transition metal, where the incident molecule first traps onto the surface and then reacts from a physisorbed molecular state, has only been observed on Ir(111) and Ir(110) at very low collision energies. We use quantum scattering methods to describe the direct reaction of methane on Ir(111) at high energy and rate theory to examine the indirect pathway at low energy. Overall, we find good agreement with the experiment with respect to the variation of sticking with the incident energy, surface temperature, and vibrational state.

Methane dissociation on stepped Ni surfaces resolved by impact site, collision energy, vibrational state, and lattice distortion.

We explore the dynamics and kinetics of methane dissociation on the steps of Ni(211) and the terraces of Ni(111), as models for step and terrace sites, respectively, on a real Ni catalyst. A quantum approach is used to compute state resolved sticking probabilities, S, and the thermally averaged sticking is computed from both S and more standard transition state methods. While the barriers can be much lower on the step edges, the terrace atoms can make important contributions to the overall reactivity if the step density is not too high and/or at higher temperatures.

Quantum dynamics studies of the dissociative chemisorption of CH on the steps and terraces of Ni(211).

The dissociative chemisorption of CH on the stepped Ni(211) surface is explored. The H and CH fragments preferentially bind to the surface along the step edge, and the barriers to dissociation are lowest over the step edge atoms, with activation energies of 0.57 and 0.

Methane dissociation on the steps and terraces of Pt(211) resolved by quantum state and impact site.

Methane dissociation on the step and terrace sites of a Pt(211) single crystal was studied by reflection absorption infrared spectroscopy (RAIRS) at a surface temperature of 120 K. The C-H stretch RAIRS signal of the chemisorbed methyl product species was used to distinguish between adsorption on step and terrace sites allowing methyl uptake to be monitored as a function of incident kinetic energy for both sites. Our results indicate a direct dissociation mechanism on both sites with higher reactivity on steps than on terraces consistent with a difference in an activation barrier height of at least 30 kJ/mol.

Surface Reaction Barriometry: Methane Dissociation on Flat and Stepped Transition-Metal Surfaces.

Accurately simulating heterogeneously catalyzed reactions requires reliable barriers for molecules reacting at defects on metal surfaces, such as steps. However, first-principles methods capable of computing these barriers to chemical accuracy have yet to be demonstrated. We show that state-resolved molecular beam experiments combined with ab initio molecular dynamics using specific reaction parameter density functional theory (SRP-DFT) can determine the molecule-metal surface interaction with the required reliability.

Water dissociation on Ni(100), Ni(110), and Ni(111) surfaces: Reaction path approach to mode selectivity.

A comparative study of mode-selectivity of water dissociation on Ni(100), Ni(110), and Ni(111) surfaces is performed at the same level of theory using a fully quantum approach based on the reaction path Hamiltonian. Calculations show that the barrier to water dissociation on the Ni(110) surface is significantly lower compared to its close-packed counterparts. Transition states for this reaction on all three surfaces involve the elongation of one of the O-H bonds.

The dissociative chemisorption of CO on Ni(100): A quantum dynamics study.

A quantum approach based on an expansion in vibrationally adiabatic eigenstates is used to explore the dissociative chemisorption of CO on Ni(100). The largest barrier to reaction corresponds to the formation of a bent anionic molecular precursor, bound to the surface by about 0.24 eV.

Quantum-state-resolved reactivity of overtone excited CH on Ni(111): Comparing experiment and theory.

Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH in three different vibrational symmetry components, A, E, and F, of the 2ν antisymmetric stretch overtone vibration as well as in the ν+ν symmetric plus antisymmetric C-H stretch combination band of F symmetry. The quantum state specific dissociation probability S (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH dissociation by Auger electron spectroscopy.

Effects of Lattice Motion on Dissociative Chemisorption: Toward a Rigorous Comparison of Theory with Molecular Beam Experiments.

The dissociative chemisorption of small molecules such as methane and water on metal surfaces is a key step in many important catalyzed reactions. However, it has only very recently become possible to directly compare theory with molecular beam studies of these reactions. For most experimental conditions, such a comparison requires accurate methods for introducing the effects of lattice motion into quantum reactive scattering calculations.

Mode-selective chemistry on metal surfaces: The dissociative chemisorption of CH4 on Pt(111).

A quantum approach based on an expansion in vibrationally adiabatic eigenstates is used to explore CH4 dissociation on Pt(111). Computed sticking probabilities for molecules in the ground, 1v3 and 2v3, states are in very good agreement with the available experimental data, reproducing the variation in reactivity with collision energy and vibrational state. As was found in similar studies on Ni(100) and Ni(111), exciting the 1v1 symmetric stretch of CH4 is more effective at promoting the dissociative chemisorption of CH4 than exciting the 1v3 antisymmetric stretch.

Lift-Off: Using Reference Imagery and Freehand Sketching to Create 3D Models in VR.

Three-dimensional modeling has long been regarded as an ideal application for virtual reality (VR), but current VR-based 3D modeling tools suffer from two problems that limit creativity and applicability: (1) the lack of control for freehand modeling, and (2) the difficulty of starting from scratch. To address these challenges, we present Lift-Off, an immersive 3D interface for creating complex models with a controlled, handcrafted style. Artists start outside of VR with 2D sketches, which are then imported and positioned in VR.

Quantum dynamics of hydrogen atoms on graphene. II. Sticking.

Following our recent system-bath modeling of the interaction between a hydrogen atom and a graphene surface [Bonfanti et al., J. Chem.

Quantum dynamics of hydrogen atoms on graphene. I. System-bath modeling.

An accurate system-bath model to investigate the quantum dynamics of hydrogen atoms chemisorbed on graphene is presented. The system comprises a hydrogen atom and the carbon atom from graphene that forms the covalent bond, and it is described by a previously developed 4D potential energy surface based on density functional theory ab initio data. The bath describes the rest of the carbon lattice and is obtained from an empirical force field through inversion of a classical equilibrium correlation function describing the hydrogen motion.

Substrate Vibrations as Promoters of Chemical Reactivity on Metal Surfaces.

Studies exploring how vibrational energy (Evib) promotes chemical reactivity most often focus on molecular reagents, leaving the role of substrate atom motion in heterogeneous interfacial chemistry underexplored. This combined theoretical and experimental study of methane dissociation on Ni(111) shows that lattice atom motion modulates the reaction barrier height during each surface atom's vibrational period, which leads to a strong variation in the reaction probability (S0) with surface temperature (Tsurf). State-resolved beam-surface scattering studies at Tsurf = 90 K show a sharp threshold in S0 at translational energy (Etrans) = 42 kJ/mol.

The dissociative chemisorption of water on Ni(111): Mode- and bond-selective chemistry on metal surfaces.

A fully quantum approach based on an expansion in vibrationally adiabatic eigenstates is used to explore the dissociative chemisorption of H2O, HOD, and D2O on Ni(111). For this late barrier system, excitation of both the bending and stretching modes significantly enhances dissociative sticking. The vibrational efficacies vary somewhat from mode-to-mode but are all relatively close to one, in contrast to methane dissociation, where the behavior is less statistical.

Dissociative chemisorption of methane on Ni and Pt surfaces: mode-specific chemistry and the effects of lattice motion.

The dissociative chemisorption of methane on metal surfaces is of great practical and fundamental interest. Not only is it the rate-limiting step in the steam re-forming of natural gas, but also the reaction exhibits interesting mode-specific behavior and a strong dependence on the temperature of the metal. Electronic structure methods are used to explore this reaction on various Ni and Pt surfaces, with a focus on how the transition state is modified by motion of the metal lattice atoms.

Dissociative chemisorption of methane on metal surfaces: tests of dynamical assumptions using quantum models and ab initio molecular dynamics.

The dissociative chemisorption of methane on metal surfaces is of great practical and fundamental importance. Not only is it the rate-limiting step in the steam reforming of natural gas, the reaction exhibits interesting mode-selective behavior and a strong dependence on the temperature of the metal. We present a quantum model for this reaction on Ni(100) and Ni(111) surfaces based on the reaction path Hamiltonian.

Ab Initio Molecular Dynamics Calculations versus Quantum-State-Resolved Experiments on CHD3 + Pt(111): New Insights into a Prototypical Gas-Surface Reaction.

The dissociative chemisorption of methane on metal surfaces is of fundamental and practical interest, being a rate-limiting step in the steam reforming process. The reaction is best modeled with quantum dynamics calculations, but these are currently not guaranteed to produce accurate results because they rely on potential energy surfaces based on untested density functionals and on untested dynamical approximations. To help overcome these limitations, here we present for the first time statistically accurate reaction probabilities obtained with ab initio molecular dynamics (AIMD) for a polyatomic gas-phase molecule reacting with a metal surface.

The dissociative chemisorption of methane on Ni(100) and Ni(111): classical and quantum studies based on the reaction path Hamiltonian.

Electronic structure methods based on density functional theory are used to construct a reaction path Hamiltonian for CH4 dissociation on the Ni(100) and Ni(111) surfaces. Both quantum and quasi-classical trajectory approaches are used to compute dissociative sticking probabilities, including all molecular degrees of freedom and the effects of lattice motion. Both approaches show a large enhancement in sticking when the incident molecule is vibrationally excited, and both can reproduce the mode specificity observed in experiments.

Quantum state-resolved CH4 dissociation on Pt(111): coverage dependent barrier heights from experiment and density functional theory.

The dissociative chemisorption of CH4 on Pt(111) was studied using quantum state-resolved methods at a surface temperature (T(s)) of 150 K where the nascent reaction products CH3(ads) and H(ads) are stable and accumulate on the surface. Most previous experimental studies of methane chemisorption on transition metal surfaces report only the initial sticking coefficients S0 on a clean surface. Reflection absorption infrared spectroscopy (RAIRS), used here for state resolved reactivity measurements, enables us to monitor the CH3(ads) uptake during molecular beam deposition as a function of incident translational energy (E(t)) and vibrational state (ν3 anti-symmetric C-H stretch of CH4) to obtain the initial sticking probability S0, the coverage dependence of the sticking probability S(θ) and the CH3(ads) saturation coverage θ(sat).