Quantum state-resolved methane scattering from Ni(111) and NiO(111) by bolometer infrared laser tagging: The effect of surface oxidation.

We describe a novel ultrahigh vacuum state-to-state molecule/surface scattering apparatus with quantum state preparation of the incident molecular beam and angle-resolved quantum state detection of the scattered molecules. State-resolved detection is accomplished using a tunable mid-infrared laser source combined with a cryogenic bolometer detector and is applicable to any molecule with an infrared-active vibrational transition. Results on rotationally inelastic scattering of CH4 methane from a Ni(111) surface and NiO(111)/Ni(111) oxide film, obtained by the new apparatus, are presented. Molecules scattering from the oxidized surface, compared to those scattering from the bare nickel surface, are more highly excited rotationally and scatter into a broader distribution of angles. The internal alignment of molecular rotation is in addition found to be stronger in molecules scattering from the bare surface. Furthermore, the maxima of the state-resolved angular distributions shift toward and away from surface normal with increasing rotational quantum number J for the oxidized and bare surface, respectively. Finally, the rotational state populations produced in scattering from the oxidized surface are well-described by a Boltzmann distribution, while those produced in scattering from the bare surface exhibit large deviations from their best-fit Boltzmann distributions. These results point toward a marked enhancement in molecule-surface collisional energy exchange induced by oxidation of the nickel surface.

Quantum state and surface-site-resolved studies of methane chemisorption by vibrational spectroscopies.

-The combination of quantum state-specific reactant preparation by infrared laser pumping with surface-site-resolved detection of chemisorbed reaction products by Reflection Absorption Infrared Spectroscopy (RAIRS) enables highly detailed studies of molecule/surface reactivity. In this perspective, we review the methodologies developed for simultaneous quantumstate and surface-site-resolved reactivity measurements and their application towards the chemisorption of methane on stepped and kinked platinum surfaces. We demonstrate that RAIRS allows for surface-site-resolved detection of methane dissociation, which serves to measure surface-site-resolved product uptake curves, sticking probabilities, and dissociation barrier heights. For the dissociation of C-H stretch excited singly deuterated CH3D on a stepped Pt surface such as Pt(211), RAIRS was used to detect bond selectivity in methane chemisorption and to reveal how the bond-selective dissociation proceeds from the step to the terrace sites with increasing incident kinetic energy of the CH3D reactant. Extension to site-selective RAIRS detection of methane dissociation to other vicinal surfaces such as Pt(210), Pt(531), and Pt(110)-(2 × 1) is also presented.

Transferability of the SRP32-vdW specific reaction parameter functional to CHD3 dissociation on Pt(110)-(2 × 1).

Stepped transition metal surfaces, including the reconstructed Pt(110)-(2 × 1) surface, can be used to model the effect of line defects on catalysts. We present a combined experimental and theoretical study of CHD3 dissociation on this surface. Theoretical predictions for the initial sticking coefficients, S0, are obtained from ab initio molecular dynamics calculations using the specific reaction parameter (SRP) approach to density functional (DF) theory, while the measured sticking coefficients were obtained using the King and Wells method. The SRP DF used here had been previously derived for methane dissociation on Pt(111) so that the experiments test the transferability of this SRP DF to methane + Pt(110)-(2 × 1). The agreement between the experimental and calculated S0 is poor, with the average energy shift between the theoretical and measured reactivities being 20 kJ/mol. There are two factors which may contribute to this difference, the first of which is that there is a large uncertainty in the calculated sticking coefficients due to a large number of molecules being trapped on the surface at the end of the 1 ps propagation time. The second is that the SRP32-vdW functional may not accurately describe the Pt(110)-(2 × 1) surface. At the lowest incident energies considered here, Pt(110)-(2 × 1) is more reactive than the flat Pt(111) surface, but the situation is reversed at incident energies above 100 kJ/mol.

Vibrational Energy Redistribution in a Gas-Surface Encounter: State-to-State Scattering of CH_{4} from Ni(111).

The fate of vibrational energy in the collision of methane (CH_{4}) in its antisymmetric C-H stretch vibration (ν_{3}) with a Ni(111) surface has been studied in a state-to-state scattering experiment. Laser excitation in the incident molecular beam prepared the J=1 rotational state of ν_{3}, and a bolometer in combination with selective laser excitation detected the scattered methane. The rovibrationally resolved scattering distributions reveal very efficient vibrational energy redistribution from ν_{3} to the symmetric C-H stretch vibration (ν_{1}). The branching ratio ν_{1}/ν_{3} is near 0.4 and insensitive to changes in incident kinetic energy in the range from 100 to 370 meV. State-resolved angular distributions and measurements of the residual Doppler linewidths prove that the scattering is direct. The observed vibrationally inelastic scattering provides direct experimental evidence for surface-induced vibrational energy redistribution.

Quantum State-Resolved Studies of Chemisorption Reactions.

Chemical reactions at the gas-surface interface are ubiquitous in the chemical industry as well as in nature. Investigating these processes at a microscopic, quantum state-resolved level helps develop a predictive understanding of this important class of reactions. In this review, we present an overview of the field of quantum state-resolved gas-surface reactivity measurements that explore the role of the initial quantum state on the dissociative chemisorption of a gas-phase reactant incident on a solid surface. Using molecular beams and either quantum state-specific reactant preparation or product detection by laser excitation, these studies have observed mode specificity and bond selectivity as well as steric effects in chemisorption reactions, highlighting the nonstatistical and complex nature of gas-surface reaction dynamics.

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.

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. State-specific preparation of incident CH4 with one quantum of antisymmetric (ν3) stretch vibration further increases the CH4 reactivity enabling comparison between translational and vibrational activation on both steps and terraces. The reaction is modeled with first principles quantum theory that accurately describes dissociative chemisorption at different sites on the surface.

Quantum-state-resolved reactivity of overtone excited CH4 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 CH4 on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH4 in three different vibrational symmetry components, A1, E, and F2, of the 2ν3 antisymmetric stretch overtone vibration as well as in the ν1+ν3 symmetric plus antisymmetric C-H stretch combination band of F2 symmetry. The quantum state specific dissociation probability S0 (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH4 dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S0 for the most reactive state ν1+ν3 is an order of magnitude higher than for the least reactive, more energetic 2ν3-E state. Our first principles quantum scattering calculations show that as molecules in the ν1 state approach the surface, the vibrational amplitude becomes localized on the reacting C-H bond, making them very reactive. This behavior results from the weakening of the reacting C-H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C-H stretches. Similarly, we find that overtone normal mode states with more ν1 character are more reactive: S0(2ν1) > S0(ν1 + ν3) > S0(2ν3). The 2ν3 eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν1 and ν1 + ν3 normal modes, being of A1 and F2 symmetry, can contribute to the 2ν3-A1 and 2ν3-F2 eigenstates, respectively, boosting their reactivity over the E component, which contains no ν1 character due to symmetry.

Quantum state resolved gas-surface reaction dynamics experiments: a tutorial review.

We present a tutorial review of our quantum state resolved experiments designed to study gas-surface reaction dynamics. The combination of a molecular beam, state specific reactant preparation by infrared laser pumping, and ultrahigh vacuum surface analysis techniques make it possible to study chemical reactivity at the gas-surface interface in unprecedented detail. We describe the experimental techniques used for state specific reactant preparation and for detection of surface bound reaction products developed in our laboratory. Using the example of the reaction of methane on Ni and Pt surfaces, we show how state resolved experiments uncovered clear evidence for vibrational mode specificity and bond selectivity, as well as steric effects in chemisorption reactions. The state resolved experimental data provides valuable benchmarks for comparison with theoretical models for gas-surface reactivity aiding in the development of a detailed microscopic understanding of chemical reactivity at the gas-surface interface.

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.

Bond-Selective and Mode-Specific Dissociation of CH3D and CH2D2 on Pt(111).

Infrared laser excitation of partially deuterated methanes (CH3D and CH2D2) in a molecular beam is used to control their dissociative chemisorption on a Pt(111) single crystal and to determine the quantum state-resolved dissociation probabilities. The exclusive detection of C-H cleavage products adsorbed on the Pt(111) surface by infrared absorption reflection spectroscopy indicates strong bond selectivity for both methane isotopologues upon C-H stretch excitation. Furthermore, the dissociative chemisorption of both methane isotopologues is observed to be mode-specific. Excitation of symmetric C-H stretch modes produces a stronger reactivity increase than excitation of the antisymmetric C-H stretch modes, whereas bend overtone excitation has a weaker effect on reactivity. The observed mode specificity and bond selectivity are rationalized by the sudden vector projection model in terms of the overlap of the reactant's normal mode vectors with the reaction coordinate at the transition state.

Quantum state specific reactant preparation in a molecular beam by rapid adiabatic passage.

Highly efficient preparation of molecules in a specific rovibrationally excited state for gas/surface reactivity measurements is achieved in a molecular beam using tunable infrared (IR) radiation from a single mode continuous wave optical parametric oscillator (cw-OPO). We demonstrate that with appropriate focusing of the IR radiation, molecules in the molecular beam crossing the fixed frequency IR field experience a Doppler tuning that can be adjusted to achieve complete population inversion of a two-level system by rapid adiabatic passage (RAP). A room temperature pyroelectric detector is used to monitor the excited fraction in the molecular beam and the population inversion is detected and quantified using IR bleaching by a second IR-OPO. The second OPO is also used for complete population transfer to an overtone or combination vibration via double resonance excitation using two spatially separated RAP processes.

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). We observe that both S0 and θ(sat) increase with increasing E(t) as well as upon ν3 excitation of the incident CH4 which indicates a coverage dependent dissociation barrier height for the dissociation of CH4 on Pt(111) at low surface temperature. This interpretation is supported by density functional calculations of barrier heights for dissociation, using large supercells containing one or more H and/or methyl adsorbates. We find a significant increase in the activation energies with coverage. These energies are used to construct simple models that reasonably reproduce the uptake data and the observed saturation coverages.

Surface-induced vibrational energy redistribution in methane/surface scattering depends on catalytic activity.

Recent state-to-state experiments of methane scattering from Ni(111) and graphene-covered Ni(111) combined with quantum mechanical simulations suggest an intriguing correlation between the surface-induced vibrational energy redistribution (SIVR) during the molecule/surface scattering event and the catalytic activity for methane dissociation of the target surface (Werdecker, Phys. Rev. Res., 2020, 2, 043251). Herein, we report new quantum state and angle-resolved measurements for methane scattering from Ni(111) and Au(111) probing the extent of ν3→ν1 antisymmetric-to-symmetric conversion of methane stretching motion for two surfaces with different catalytic activities. Consistent with the expectations, the extent of SIVR occurring on the more catalytically active Ni(111) surface, as measured by the ν1:ν3 scattered population ratio, is found to be several times stronger than that on the more inert Au(111) surface. We also present additional insights on the rovibrational scattering dynamics contained in the angle- and state-resolved data. The results together highlight the power of state-resolved scattering measurements as a tool for investigating methane-surface interactions.

Probing Water Dissociation and Oxygen Replacement on Partially Oxygen-Covered Cu(111) by Reflection Absorption Infrared Spectroscopy.

The presence of chemisorbed oxygen on the Cu(111) surface is known to strongly reduce the activation barrier for water dissociation as compared to bare Cu(111). Here, we present direct experimental evidence for the hydrogen abstraction mechanism responsible for the facile H2O dissociation on an O/Cu(111) surface using reflection absorption infrared spectroscopy (RAIRS) in combination with isotopically labeled reactants. We also observe that chemisorbed hydroxyl species produced by water dissociation on the O/Cu(111) surface undergo an efficient hydrogen atom transfer from trapped water molecules, leading to the rapid replacement of the initial oxygen isotope coverage and the detection of only a single hydroxyl isotopologue on the surface, in apparent contradiction with the hydrogen abstraction mechanism. In the presence of Cu2O oxide islands on the O/Cu(111) surface, water dissociation occurs selectively at the edges of those islands, leading to the self-assembly of isotopically ordered structures.

The sticking probability of D2O-water on ice: isotope effects and the influence of vibrational excitation.

The present study measures the sticking probability of heavy water (D(2)O) on H(2)O- and on D(2)O-ice and probes the influence of selective OD-stretch excitation on D(2)O sticking on these ices. Molecular beam techniques are combined with infrared laser excitation to allow for precise control of incident angle, translational energy, and vibrational state of the incident molecules. For a translational energy of 69 kJ/mol and large incident angles (θ ≥ 45°), the sticking probability of D(2)O on H(2)O-ice was found to be 1% lower than on D(2)O-ice. OD-stretch excitation by IR laser pumping of the incident D(2)O molecules produces no detectable change of the D(2)O sticking probability (<10(-3)). The results are compared with other gas/surface systems for which the effect of vibrational excitation on trapping has been probed experimentally.

Methylidyne Adsorption on Pt(211) Probed by Reflection Absorption Infrared Spectroscopy (RAIRS).

Methylidyne, CH(ads), adsorbed on a Pt(211) surface and its interaction with chemisorbed hydrogen atoms was studied by reflection absorption infrared spectroscopy (RAIRS). Methylidyne was formed on Pt(211) by methane dissociation from a molecular beam followed by thermal decomposition of the methane dissociation products. CH(ads) was detected by RAIRS via its symmetric C-H stretch vibration resulting in three discrete absorption peaks in the region of 2950-2970 cm-1. While the frequencies of the three C-H stretch peaks remain fixed, their relative intensities depend on the H(ads) co-coverage. This differs markedly from what was observed previously for the RAIR spectra of CH(ads) adsorbed on Pt(111) by the group of Trenary,1 who observed a single C-H stretch peak, which showed a continuous blue shift with increasing H(ads) coverage. Based on our experimental results and density functional theory (DFT) calculations, we propose that the three discrete absorption peaks on Pt(211) are due to the adsorption of methylidyne on the steps of Pt(211) forming one-dimensional rows of adsorbates. Depending on the H(ads) coverage, the CH(ads) species on the step sites can have either zero, one, or two neighboring H(ads) atoms, leading to three different vibrational C-H stretch frequencies and a reversible shift in relative peak intensity depending on the H(ads) coverage.

Alignment dependent chemisorption of vibrationally excited CH4(ν3) on Ni(100), Ni(110), and Ni(111).

We present a stereodynamics study of the dissociative chemisorption of vibrationally excited methane on the (100), (110), and (111) planes of a nickel single crystal surface. Using linearly polarized infrared excitation of the antisymmetric C-H stretch normal mode vibration (ν(3)), we aligned the angular momentum and C-H stretch amplitude of CH(4)(ν(3)) in the laboratory frame and measured the alignment dependence of state-resolved reactivity of CH(4) for the ν(3) = 1, J = 0-3 quantum states over a range of incident translational energies. For all three surfaces studied, in-plane alignment of the C-H stretch results in the highest dissociation probability and alignment along the surface normal in the lowest reactivity. The largest alignment contrast between the maximum and minimum reactivity is observed for Ni(110), which has its surface atoms arranged in close-packed rows separated by one layer deep troughs. For Ni(110), we also probed for alignment effects relative to the direction of the Ni rows. In-plane C-H stretch alignment perpendicular to the surface rows results in higher reactivity than parallel to the surface rows. The alignment effects on Ni(110) and Ni(100) are independent of incident translational energy between 10 and 50 kJ/mol. Quantum state-resolved reaction probabilities are reported for CH(4)(ν(3)) on Ni(110) for translational energies between 10 and 50 kJ/mol.

Infrared Activities of Adsorbed Species on Metal Surfaces: The Puzzle of Adsorbed Methyl (CH3).

Reflection-absorption infrared spectroscopy (RAIRS) is widely used to identify molecular adsorbates on metals during surface chemical reactions, but the interpretation of RAIRS data can be difficult with experiment alone. Here, we reveal from first-principles calculations the origin of the contrasting RAIRS spectra of methyl adsorbed on Pt(111) and Ni(111). We find that the dynamic dipole associated with the symmetric C-H stretch vibration of CH3 along surface normal is significant on Pt(111) but negligibly small on Ni(111), explaining the strong IR activity in the former and the absence of any RAIRS peaks in the latter. This difference is correlated to different charge transfer patterns between metals and the adsorbate, which are determined by the different preferred adsorption sites of methyl on the two surfaces. This work highlights the need of electronic structure calculations in interpreting RAIRS spectra of adsorbates on metal surfaces.