Incident Translational Energy Research Articles

A Linearized Semiclassical Dynamics Study of the Multiquantum Vibrational Relaxation of NO Scattering from a Au(111) Surface.

The vibrational relaxation of NO molecules scattering from a Au(111) surface has served as the focus of efforts to understand nonadiabatic energy transfer at metal-molecule interfaces. Experimental measurements and previous theoretical efforts suggest that multiquantal NO vibrational energy relaxation occurs via electron-hole pair excitations in the metal. Here, using a linearized semiclassical approach, we accurately predict the vibrational relaxation of NO from the νi = 3 state for different incident translational energies. We also accurately capture the central role of transient electron transfer from the metal to the molecule in mediating the vibrational relaxation process but fall short of quantitatively predicting the full extent of multiquantum relaxation for high incident vibrational excitations (νi = 16).

Sensitivity Analysis of the Catalysis Recombination Mechanism on Nanoscale Silica Surfaces.

A deep understanding of surface catalysis recombination characteristics is significant for accurately predicting the aeroheating between hypersonic non-equilibrium flow and thermal protection materials, while a de-coupling sensitivity analysis of various influential factors is still lacking. A gas–solid interface (GSI) model with a hyperthermal flux boundary was established to investigate the surface catalysis recombination mechanisms on nanoscale silica surfaces. Using the reactive molecular dynamics (RMD) simulation method, the effects of solid surface temperature, gas incident angle, and translational energy on the silica surface catalysis recombination were qualified under hyperthermal atomic oxygen (AO), atomic nitrogen (AN), and various AN/AO gas mixtures’ influence. It can be found that, though the Eley–Rideal (E–R) recombination mechanism plays a dominant role over the Langmuir–Hinshelwood (L–H) mechanism for all the sensitivity analyses, a non-linear increasing pattern of AO recombination coefficient γO2 with the increase in incident angle θin and translational energy Ek is observed. Compared with the surface catalysis under hyperthermal AO impact, the AN surface adsorption fraction shows an inverse trend with the increase in surface temperature, which suggests the potential inadequacy of the traditional proportional relationship assumptions between the surface adsorption concentration and the surface catalysis recombination coefficient for other species’ impact instead of AOs. For the incoming bi-component AO/AN gas mixtures, the corresponding surface catalysis coefficient is not the simple superposition of the effects of individual gases but is affected by both the intramolecular bond energies (e.g., O2, N2) and intermolecular energies (e.g., Si/N, Si/O).

Inelastic scattering dynamics of naphthalene and 2-octanone on highly oriented pyrolytic graphite.

The inelastic scattering dynamics of the isobaric molecules, naphthalene (C10H8) and 2-octanone (C8H16O), on highly oriented pyrolytic graphite (HOPG) have been investigated as part of a broader effort to inform the inlet design of a mass spectrometer for the analysis of atmospheric gases during a flyby mission through the atmosphere of a planet or moon. Molecular beam-surface scattering experiments were conducted, and the scattered products were detected with the use of a rotatable mass spectrometer detector. Continuous, supersonic beams were prepared, with average incident translational energies, ⟨Ei⟩, of 247.3 kJ mol-1 and 538.2 kJ mol-1 for naphthalene and 268.6 kJ mol-1 and 433.8 kJ mol-1 for 2-octanone. These beams were directed toward an HOPG surface, held at 530 K, at incident angles, θi, of 30°, 45°, and 70°, and scattered products were detected as functions of their translational energies and scattering angles. The scattering dynamics of both molecules are very similar and mimic the scattering of atoms and small molecules on rough surfaces, where parallel momentum is not conserved, suggesting that the dynamics are dominated by a corrugated interaction potential between the incident molecule and the surface. The effective corrugation of the molecule-surface interaction is apparently caused by the structure of the incident molecule and the consequent myriad available energy transfer pathways between the molecule and the surface during a complex collision event. In addition, the HOPG surface contributes to the corrugation of the interaction potential because it can absorb significant energy from collisions with incident molecules that have high mass and incident energy. Small differences in the scattering dynamics of the two molecules are inferred to arise from the details of the molecule-surface interaction potential, with 2-octanone exhibiting dynamics that suggest a slightly stronger interaction with the surface than naphthalene. These results add to a growing body of work on the scattering dynamics of organic molecules on HOPG, from which insight into the hypervelocity sampling and analysis of such molecules may be obtained.

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

Efficient transfer of translational energy to electron-hole pair excitation involving multiple collisions dominates H atom collisions with metal surfaces. For this reason, H atom interaction with metal surfaces cannot be modeled within the commonly used Born-Oppenheimer approximation (BOA). This fact makes H atom scattering from metal surfaces an ideal model system for dynamics that go beyond the BOA. We chose the H/Au(111) system as a model system to obtain a detailed dataset that can serve as a benchmark for theoretical models developed for describing electronically nonadiabatic processes at metal surfaces. Therefore, we investigate the influence of various experimental parameters on the energy loss in detail including isotopic variant, incidence translational energy, incidence polar and azimuthal angles, and outgoing scattering angles.

Site Selective Detection of Methane Dissociation on Stepped Pt Surfaces

We report a combined experimental and theoretical study comparing methane dissociation on three different platinum surfaces Pt(111), Pt(211), and Pt(110)-(1 × 2). Reflection absorption infrared spectroscopy (RAIRS) was used to detect chemisorbed methyl species formed by dissociative chemisorption of CH4 on specific surface sites and to measure surface-site-specific sticking coefficients of CH4 on the terrace, step, and ridge sites as function of incident translational energy. Methane dissociation is observed to be direct on all sites and diffusion of the chemisorbed methyl species is absent for surface temperature below 150 K. The experimental data are compared with the results of density functional (DFT) calculations that give minimum energy barriers for CH4 chemisorption that properly account for the experimental relative site-specific reactivities. Also in agreement with experiments, DFT results predict a negligible effect of co-adsorbed H and CH3 species on the vibrational frequency of a methyl group chemisorbed on terrace and step sites of Pt(211). However, the origin of the red-shift of the RAIRS peak of CH3 chemisorbed on terrace sites compared with that on step sites of Pt(211) remains elusive and still demands further investigation.

Bond selective dissociation of methane (CH3D) on the steps and terraces of Pt(211).

The dissociative chemisorption of singly deuterated methane (CH3D) has been studied on the steps and terraces of a Pt(211) surface by quantum state resolved molecular beam methods. At incident translational energy (Et) below 50 kJ/mol, CH3D dissociates only on the more reactive steps of Pt(211), where both C-H and C-D cleavage products CH2D(ads) and CH3(ads) can be detected by reflection absorption infrared spectroscopy. Vibrational excitation of a slow beam of CH3D (Et = 10 kJ/mol), prepared with one quantum of antisymmetric C-H stretch excitation by infrared laser pumping, allows for fully bond- and site-selective dissociation forming exclusively CH2D(ads) on the step sites. At higher kinetic energies (Et > 30 kJ/mol), bond selective dissociation by C-H bond cleavage is observed on the terrace sites for stretch excited CH3D (ν4) while on the steps, the C-H/C-D cleavage branching ratio approaches the statistical 3/1 limit. Finally, at Et > 60 kJ/mol, both C-H and C-D cleavages are observed on both step and terrace sites of Pt(211). Our experiments show how careful control of incident translational and vibrational energy can be used for site and bond selective dissociation of methane on a catalytically active Pt surface.

Scattering Dynamics of Nitromethane and Methyl Formate on Highly Oriented Pyrolytic Graphite (HOPG)

The gas–surface scattering dynamics of nitromethane (CH3NO2) and methyl formate (HCOOCH3) on a highly oriented pyrolytic graphite (HOPG) surface have been investigated as part of a broader effort to evaluate the efficacy of a funnel-like neutral gas concentrator that has been proposed as a mass spectrometer inlet for the characterization of tenuous planetary atmospheres or plumes. Molecular beams of CH3NO2 and HCOOCH3 with incidence energies, Ei, of 106.5 and 98.8 kJ mol–1, respectively, were directed at the surface with incidence angles, θi, of 70, 45, and 30°. A rotatable mass spectrometer, employing electron-impact ionization, was used to collect angle-resolved time-of-flight (TOF) distributions of the molecules that scattered inelastically from the surface, allowing angular distributions of the scattered product flux and translational energy distributions at a given final angle, θf, to be obtained. The TOF distributions of the scattered products detected the parent ion mass-to-charge ratios and their respective dominant ion fragments were identical, indicating that CH3NO2 and HCOOCH3 fragmented in the ionizer of the detector and not while colliding with the surface. The scattering dynamics suggested that the parallel momentum of the molecules was conserved during impact with the surface. The translational energy and angular distributions of CH3NO2 and HCOOCH3 were identical when θi = 70°. For θi = 45 and 30°, the HCOOCH3 angular distributions were shifted to a slightly larger θf than the CH3NO2 distributions. The molecules scattered from the surface through impulsive scattering (IS) and quasitrapping (QT) pathways. The IS molecules retained a large fraction of their incidence translational energy when colliding with the surface. The QT molecules transferred more energy, but they did not come completely into thermal equilibrium with the surface before scattering into the vacuum. The QT molecules had a lobular angular distribution with a maximum flux far from the surface normal, indicating that they retained some memory of their incident conditions despite losing a significant amount of energy at the surface. The results presented in this article demonstrate that for Ei near 100 kJ mol–1, these molecules would not dissociate upon impact with the surfaces of a gas concentrator constructed of HOPG. Although the observed scattering dynamics suggest that such a concentrator could perform well for a variety of molecular species, accurate concentration factors are ultimately molecule-specific and determined by the details of the molecule–surface interaction potential.

Associative desorption of hydrogen isotopologues from copper surfaces: Characterization of two reaction mechanisms.

We report quantum-state resolved measurements of angular and velocity distributions of the associative desorption of H2, HD, and D2 from Cu(111) and Cu(211) surfaces. The desorbing molecules have bimodal velocity distributions comprising a "fast" channel and a "slow" channel on both facets. The "fast channel" is promoted by both hydrogen incidence translational and vibrational energy, while the "slow channel" is promoted by vibrational energy but inhibited by translational energy. Using detailed balance, we determine state-specific reaction probabilities for dissociative adsorption and compare these to theoretical calculations. The results for the activation barrier for the "fast channel" on Cu(111) are in agreement with theory within "chemical accuracy" (1 kcal/mole). Results on the Cu(211) facet provide direct information on the effect of increasing step density, which is commonly believed to increase reactivity. Differences in reactivity on the (111) and (211) facets are subtle - quantum state specific reactivity on the (211) surface is characterized by a broader distribution of barrier heights whose average values are higher than for reaction on (111). We fully characterize the "slow channel," which has not been found in theoretical calculations although it makes up a large fraction of the reactivity in these experiments.

Shock Energy Conversion from Translation to Rotation

To protect structures from short duration shock load in various engineering applications, a novel energy conversion mechanism with concept design is proposed. Different from conventional methods with cellular solid/structure dissipating input translational kinetic energy to plastic strain energy with large compressive deformation, the proposed approach converts part of incident translational kinetic energy to rotational kinetic energy, which is not detrimental to the protected structure. The mechanism of energy conversion is analyzed and formulated, with key factors governing the conversion efficiency identified and discussed, which sheds light on alternative approach for short duration load mitigation.

Quantum state resolved molecular beam reflectivity measurements: CH4 dissociation on Pt(111).

The King and Wells molecular beam reflectivity method has been used for a quantum state resolved study of the dissociative chemisorption of CH4 on Pt(111) at several surface temperatures. Initial sticking coefficients S0 were measured for incident CH4 prepared both with a single quantum of ν3 antisymmetric stretch vibration by infrared laser pumping and without laser excitation. Vibrational excitation of the ν3 mode is observed to be less efficient than incident translational energy in promoting the dissociation reaction with a vibrational efficacy ην3 = 0.65. The initial state resolved sticking coefficient S0ν3 was found to be independent of the surface temperature over the 50 kJ/mol to 120 kJ/mol translational energy range studied here. However, the surface temperature dependence of the King and Wells data reveals the migration of adsorbed carbon formed by CH4 dissociation on the Pt(111) surface leading to the growth of carbon particles.

Reactive-Atom Scattering from Liquid Crystals at the Liquid-Vacuum Interface: [C12mim][BF4] and 4-Cyano-4'-Octylbiphenyl (8CB).

Two complementary approaches were used to study the liquid-vacuum interface of the liquid-crystalline ionic liquid 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12mim][BF4]) in the smectic A (SmA) and isotropic phases. O atoms with two distinct incident translational energies were scattered from the surface of [C12mim][BF4]. Angle-dependent time-of-flight distributions and OH yields, respectively, were recorded from high- and low-energy O atoms. There were no significant changes in the measurements using either approach, nor the properties derived from them, accompanying the transition from the SmA to the isotropic phase. This indicates that the surface structure of [C12mim][BF4] remains essentially unchanged across the phase boundary, implying that the bulk order and surface structure are not strongly correlated for this material. This effect is ascribed to the strong propensity for the outer surfaces of ionic liquids to be dominated by alkyl chains, over an underlying layer rich in anions and cation head groups, whether or not the bulk material is a liquid crystal. In a comparative study, the OH yield from the surface of the liquid crystal, 8CB, was found to be affected by the bulk order, showing a surprising step increase at the SmA-nematic transition temperature, whose origin is the subject of speculation.

Vibrational energy transfer near a dissociative adsorption transition state: State-to-state study of HCl collisions at Au(111).

In this work we seek to examine the nature of collisional energy transfer between HCl and Au(111) for nonreactive scattering events that sample geometries near the transition state for dissociative adsorption by varying both the vibrational and translational energy of the incident HCl molecules in the range near the dissociation barrier. Specifically, we report absolute vibrational excitation probabilities for HCl(v = 0 → 1) and HCl(v = 1 → 2) scattering from clean Au(111) as a function of surface temperature and incidence translational energy. The HCl(v = 2 → 3) channel could not be observed-presumably due to the onset of dissociation. The excitation probabilities can be decomposed into adiabatic and nonadiabatic contributions. We find that both contributions strongly increase with incidence vibrational state by a factor of 24 and 9, respectively. This suggests that V-T as well as V-EHP coupling can be enhanced near the transition state for dissociative adsorption at a metal surface. We also show that previously reported HCl(v = 0 → 1) excitation probabilities [Q. Ran et al., Phys. Rev. Lett. 98, 237601 (2007)]-50 times smaller than those reported here-were influenced by erroneous assignment of spectroscopic lines used in the data analysis.

Scattering Dynamics, Survival, and Dispersal of Dimethyl Methylphosphonate Interacting with the Surface of Multilayer Graphene.

We explored the interaction of a molecular beam of dimethyl methylphosphonate with a multilayer graphene surface to better understand the fate of chemical warfare agents in the environment. The experiments were done at surface temperatures between 120 and 900 K and translational energies between 200 and 1500 meV. At the lowest temperatures, the dimethyl methylphosphonate is adsorbed, with the molecules next to the carbon surface held slightly more strongly than the bulk molecular film that grows with continued dosing. We measured the desorption energy for submonolayer coverage using modulated beam techniques and found a value of 290 meV (28 kJ/mol). At higher surface temperatures, where the residence times are very short, we measured the scattering of the dimethyl methylphosphonate as a function of angle and translational kinetic energy. For a surface temperature of 250 K, with translational kinetic energies between 200 and 1500 meV, much of the incident flux has nearly been accommodated by the surface temperature and has no memory of the incident momentum. The internal energy also seems to be at least partially accommodated. As the surface temperature increases, the scattering transitions to direct-inelastic reflection, where much of the incident translational energy is retained, and the intensity of the scattering peaks superspecularly toward glancing final angles. These results demonstrate the efficacy of using kinetic energy controlled molecular beams to probe the interactions of complex organic molecules with well-defined surfaces, extending our fundamental understanding of how the dynamics for such systems crossover from trapping-desorption to direct inelastic scattering. Moreover, these results indicate that simulations that model the dispersal of chemical warfare agents using common interfaces in the environment need to account for multiple bounce trajectories and survival of the impinging molecules.

NO Vibrational Energy Transfer on a Metal Surface: Still a Challenge to First-Principles Theory

During a collision of highly vibrationally excited NO with a Au(111) surface, the molecule can lose a large fraction of its vibrational energy into electronic excitation of the metal. This process violates the Born–Oppenheimer approximation and represents a major challenge to theories of molecule–surface interaction. Two ab initio approaches to this problem, one using independent electron surface hopping (IESH) and the other electronic friction, previously reported good agreement with the limited available data on multiquantum vibrational relaxation; however, at that time only experiments for NO(vi = 15) at an incidence translational energy of Ei = 0.05 eV were available. In this work, we report a comparison of recently reported experiments characterizing the multiquantum vibrational relaxation of NO on Au(111) for a wider range of incidence translational and vibrational energies to IESH and molecular dynamics with electronic friction (MDEF) calculations for these conditions. Both theories fail to explain the large amount of vibrational energy transferred from NO to the solid.

Two-dimensional quantum dynamics of O2dissociative adsorption on Ag(111)

We have investigated the quantum dynamics of O2 dissociative adsorption on a Ag(111) surface. We performed the calculations with a Hamiltonian where the O2 translational motion is perpendicular to the surface and for O2 vibrational energy. We found that dissociative adsorption occurs with an incident translational energy below the expected activation barrier, while the translational-energy dependence for adsorption probabilities is a smooth sigmoid. Thus, there are non-negligible tunneling effects in the dissociative adsorption that are affected by the activation barrier width. Moreover, the incident translational energies at the inflection points of the adsorption probabilities shift lower with an increase in vibrational quantum numbers of the incident O2. Thus, there is significant energy transfer and coupling from vibration to translational motion. The vibrational energy assists the O2 dissociative adsorption via a vibrationally assisted sticking effect.

CO (a3Π) quenching at a metal surface: Evidence of an electron transfer mediated mechanism.

We observe a strong influence of molecular vibration and surface temperature on electron emission promoted by the de-excitation of metastable CO(a(3)Π) on a clean Au(111) surface using a molecular beam surface scattering apparatus. The de-excitation is independent of incidence translational energy. These observations appear incompatible with existing theories of metastable particle de-excitation on metal surfaces, which are based on the Auger effect. Instead, they strongly suggest a mechanism involving formation of a transient anion whose lifetime is similar to the vibrational period of the CO molecule.

Dynamical steering in an electron transfer surface reaction: Oriented NO(v = 3, 0.08 < Ei < 0.89 eV) relaxation in collisions with a Au(111) surface

We report measurements of the incidence translational energy dependence of steric effects in collisions of NO(v = 3) molecules with a Au(111) surface using a recently developed technique to orient beams of vibrationally excited NO molecules at incidence energies of translation between 0.08 and 0.89 eV. Incidence orientation dependent vibrational state distributions of scattered molecules are detected by means of resonance enhanced multiphoton ionization spectroscopy. Molecules oriented with the N-end towards the surface exhibit a higher vibrational relaxation probability than those oriented with the O-end towards the surface. This strong orientation dependence arises from the orientation dependence of the underlying electron transfer reaction responsible for the vibrational relaxation. At reduced incidence translational energy, we observe a reduced steric effect. This reflects dynamical steering and re-orientation of the NO molecule upon its approach to the surface.

The importance of accurate adiabatic interaction potentials for the correct description of electronically nonadiabatic vibrational energy transfer: a combined experimental and theoretical study of NO(v = 3) collisions with a Au(111) surface.

We present a combined experimental and theoretical study of NO(v = 3 → 3, 2, 1) scattering from a Au(111) surface at incidence translational energies ranging from 0.1 to 1.2 eV. Experimentally, molecular beam-surface scattering is combined with vibrational overtone pumping and quantum-state selective detection of the recoiling molecules. Theoretically, we employ a recently developed first-principles approach, which employs an Independent Electron Surface Hopping (IESH) algorithm to model the nonadiabatic dynamics on a Newns-Anderson Hamiltonian derived from density functional theory. This approach has been successful when compared to previously reported NO/Au scattering data. The experiments presented here show that vibrational relaxation probabilities increase with incidence energy of translation. The theoretical simulations incorrectly predict high relaxation probabilities at low incidence translational energy. We show that this behavior originates from trajectories exhibiting multiple bounces at the surface, associated with deeper penetration and favored (N-down) molecular orientation, resulting in a higher average number of electronic hops and thus stronger vibrational relaxation. The experimentally observed narrow angular distributions suggest that mainly single-bounce collisions are important. Restricting the simulations by selecting only single-bounce trajectories improves agreement with experiment. The multiple bounce artifacts discovered in this work are also present in simulations employing electronic friction and even for electronically adiabatic simulations, meaning they are not a direct result of the IESH algorithm. This work demonstrates how even subtle errors in the adiabatic interaction potential, especially those that influence the interaction time of the molecule with the surface, can lead to an incorrect description of electronically nonadiabatic vibrational energy transfer in molecule-surface collisions.

Incidence energy dependent state-to-state time-of-flight measurements of NO(v = 3) collisions with Au(111): the fate of incidence vibrational and translational energy

We report measurements of translational energy distributions when scattering NO(vi = 3, Ji = 1.5) from a Au(111) surface into vibrational states vf = 1, 2, 3 and rotational states up to Jf = 32.5 for various incidence energies ranging from 0.11 eV to 0.98 eV. We observed that the vibration-to-translation as well as the translation-to-rotation coupling depend on translational incidence energy, EI. The vibration-to-translation coupling, i.e. the additional recoil energy observed for vibrationally inelastic (v = 3 → 2, 1) scattering, is seen to increase with increasing EI. The final translational energy decreases approximately linearly with increasing rotational excitation. At incidence energies EI > 0.5 eV, the slopes of these dependencies are constant and identical for the three vibrational channels. At lower incidence energies, the slopes gradually approach zero for the vibrationally elastic channel while they exhibit more abrupt transitions for the vibrationally inelastic channels. We discuss possible mechanisms for both effects within the context of nonadiabatic electron-hole pair mediated energy transfer and orientation effects.

State-to-State Time-of-Flight Measurements of NO Scattering from Au(111): Direct Observation of Translation-to-Vibration Coupling in Electronically Nonadiabatic Energy Transfer

Translational motion is believed to be a spectator degree of freedom in electronically nonadiabatic vibrational energy transfer between molecules and metal surfaces, but the experimental evidence available to support this view is limited. In this work, we have experimentally determined the translational inelasticity in collisions of NO molecules with a single-crystal Au(111) surface-a system with strong electronic nonadiabaticity. State-to-state molecular beam surface scattering was combined with an IR-UV double resonance scheme to obtain high-resolution time-of-flight data. The measurements include vibrationally elastic collisions (v = 3→3, 2→2) as well as collisions where one or two quanta of molecular vibration are excited (2→3, 2→4) or de-excited (2→1, 3→2, 3→1). In addition, we have carried out comprehensive measurements of the effects of rotational excitation on the translational energy of the scattered molecules. We find that under all conditions of this work, the NO molecules lose a large fraction (∼0.45) of their incidence translational energy to the surface. Those molecules that undergo vibrational excitation (relaxation) during the collision recoil slightly slower (faster) than vibrationally elastically scattered molecules. The amount of translational energy change depends on the surface temperature. The translation-to-rotation coupling, which is well-known for v = 0→0 collisions, is found to be significantly weaker for vibrationally inelastic than elastic channels. Our results clearly show that the spectator view of the translational motion in electronically nonadiabatic vibrational energy transfer between NO and Au(111) is only approximately correct.