Bruce L. Yoder

Steric Effects in the Chemisorption of Vibrationally Excited Methane on Ni(100)

Tilting Toward Reaction Collisions between molecules and metal surfaces underlie many of the catalytic pathways that transform natural feedstocks into fuels and commodity chemical compounds. One such reaction, in which nickel strips hydrogen from methane, depends on whether the methyl C-H bonds are vibrating just before the molecule strikes the surface. Yoder et al. (p. 553) now delve deeper into this system. By aligning incoming molecular samples using polarized infrared light, they show that the hydrocarbon reacts most readily when it is vibrating parallel, rather than perpendicular, to the surface. The reaction underlying industrial hydrogen production depends subtly on the reagent’s orientation toward the catalyst. Newly available, powerful infrared laser sources enable the preparation of intense molecular beams of quantum-state prepared and aligned molecules for gas/surface reaction dynamics experiments. We present a stereodynamics study of the chemisorption of vibrationally excited methane on the (100) surface of nickel. Using linearly polarized infrared excitation of the C-H stretch modes of two methane isotopologues [CH4(ν3) and CD3H(ν1)], we aligned methane’s angular momentum and vibrational transition dipole moment in the laboratory frame. An increase in methane reactivity of as much as 60% is observed when the laser polarization is parallel rather than normal to the surface. The dependence of the alignment effect on the rotational branch used for excitation indicates that alignment of the vibrational transition dipole moment of methane is responsible for the steric effect. Potential explanations for the steric effect in terms of an alignment-dependent reaction barrier height or electronically nonadiabatic effects are discussed.

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.

Barrierless proton transfer across weak CH⋯O hydrogen bonds in dimethyl ether dimer

We present a combined computational and threshold photoelectron photoion coincidence study of two isotopologues of dimethyl ether, (DME - h6)n and (DME - d6)n n = 1 and 2, in the 9-14 eV photon energy range. Multiple isomers of neutral dimethyl ether dimer were considered, all of which may be present, and exhibited varying C-H⋯O interactions. Results from electronic structure calculations predict that all of them undergo barrierless proton transfer upon photoionization to the ground electronic state of the cation. In fact, all neutral isomers were found to relax to the same radical cation structure. The lowest energy dissociative photoionization channel of the dimer leads to CH3OHCH3 (+) by the loss of CH2OCH3 with a 0 K appearance energy of 9.71 ± 0.03 eV and 9.73 ± 0.03 eV for (DME - h6)2 and deuterated (DME - d6)2, respectively. The ground state threshold photoelectron spectrum band of the dimethyl ether dimer is broad and exhibits no vibrational structure. Dimerization results in a 350 meV decrease of the valence band appearance energy, a 140 meV decrease of the band maximum, thus an almost twofold increase in the ground state band width, compared with DME - d6 monomer.

Electron mean free path from angle-dependent photoelectron spectroscopy of aerosol particles

We propose angle-resolved photoelectron spectroscopy of aerosol particles as an alternative way to determine the electron mean free path of low energy electrons in solid and liquid materials. The mean free path is obtained from fits of simulated photoemission images to experimental ones over a broad range of different aerosol particle sizes. The principal advantage of the aerosol approach is twofold. First, aerosol photoemission studies can be performed for many different materials, including liquids. Second, the size-dependent anisotropy of the photoelectrons can be exploited in addition to size-dependent changes in their kinetic energy. These finite size effects depend in different ways on the mean free path and thus provide more information on the mean free path than corresponding liquid jet, thin film, or bulk data. The present contribution is a proof of principle employing a simple model for the photoemission of electrons and preliminary experimental data for potassium chloride aerosol particles.

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.

Angle-Resolved Photoemission of Solvated Electrons in Sodium-Doped Clusters.

Angle-resolved photoelectron spectroscopy of the unpaired electron in sodium-doped water, methanol, ammonia, and dimethyl ether clusters is presented. The experimental observations and the complementary calculations are consistent with surface electrons for the cluster size range studied. Evidence against internally solvated electrons is provided by the photoelectron angular distribution. The trends in the ionization energies seem to be mainly determined by the degree of hydrogen bonding in the solvent and the solvation of the ion core. The onset ionization energies of water and methanol clusters do not level off at small cluster sizes but decrease slightly with increasing cluster size.

Angle-resolved valence shell photoelectron spectroscopy of neutral nanosized molecular aggregates

This mini-review provides an overview of the recent developments in the field of angle-resolved photoelectron spectroscopy of neutral weakly-bound molecular aggregates with sizes that range from the gas phase monomer into the lower nanometre regime. A summary of the recent first size-dependent studies of neutral single-component and solute/solvent nano-clusters in the valence-shell region is provided. The challenges in determining independent information on the particle size as well as accurate angular-dependent information are highlighted. Examples of potential artefacts that falsify the true photoelectron angular distribution are discussed.

Simple Relationship between Oxidation State and Electron Affinity in Gas-Phase Metal–Oxo Complexes

The photoelectron spectra of WO3H(-) and WO2F(-) are presented and analyzed in the context of a series of previous similar measurements on MO(y)(-) (M = Mo, W; y = 0-3), MO4H(-) and AlMOy(-) (y ≤ 4) complexes. The electronic structures of the WO3H and WO2F anion and neutral complexes were investigated using the B3LYP hybrid density functional method. The spectra of WO3H(-), WO2F(-), and previously measured AlWO3(-) photoelectron spectra show that the corresponding neutrals, in which the transition metal centers are all in a +5 oxidation state, have comparable electron affinities. In addition, the electron affinities fit the general trend of monotonically increasing electron affinity with oxidation state, in spite of the WO3H(-), WO2F(-), and AlWO3(-) having closed shell ground states, suggesting that the oxidation state of the metal atom has more influence than shell closing on the electron affinity of these transition metal-oxo complexes. Results of DFT calculations suggest that the neutrals are pyramidal and the anions are planar. However, the barriers for inversion on the neutral surface are low, and attempts to generate simple Franck-Condon simulations based on simple normal coordinate displacement, ignoring the effects of inversion, are inadequate.

Solvated Electrons in Clusters: Magic Numbers for the Photoelectron Anisotropy

This paper reports on a curiosity concerning magic numbers in neutral molecular clusters, namely on magic numbers related to the photoelectron anisotropy in angle-resolved photoelectron spectra. With a combination of density functional calculations and experiment, we search for magic numbers in Na(H2O)n, Na(NH3)n, Na(CH3OH)n, and Na(CH3OCH3)n clusters. In clusters of high symmetry, the highest occupied molecular orbital can be delocalized over an extended region, forming a symmetric charge distribution of high s character, which results in a pronounced anisotropy in the photoelectron angular distribution. We find magic numbers at n = 6 and 4 for sodium-doped dimethyl ether and ammonia clusters, respectively, but not for sodium-doped water and methanol clusters, which is likely a consequence of the degree of hydrogen bonding and the number of structural isomers.

Size-Resolved Photoelectron Anisotropy of Gas Phase Water Clusters and Predictions for Liquid Water.

We report the first measurements of size-resolved photoelectron angular distributions for the valence orbitals of neutral water clusters with up to 20 molecules. A systematic decrease of the photoelectron anisotropy is found for clusters with up to 5-6 molecules, and most remarkably, convergence of the anisotropy for larger clusters. We suggest the latter to be the result of a local short-range scattering potential that is fully described by a unit of 5-6 molecules. The cluster data and a detailed electron scattering model are used to predict the anisotropy of slow photoelectrons in liquid water. Reasonable agreement with experimental liquid jet data is found.