William A. Alexander

Kinematics and dynamics of atomic-beam scattering on liquid and self-assembled monolayer surfaces

We have conducted investigations of the energy transfer dynamics of atomic oxygen and argon scattering from hydrocarbon and fluorocarbon surfaces. In light of these results, we appraise the applicability and value of a kinematic scattering model, which views a gas-surface interaction as a gas-phase-like collision between an incident atom or molecule and a localized region of the surface with an effective mass. We have applied this model to interpret the effective surface mass and energy transfer when atoms strike two different surfaces under identical bombardment conditions. To this end, we have collected new data, and we have re-examined existing data sets from both molecular-beam experiments and molecular dynamics simulations. We seek to identify trends that could lead to a robust general understanding of energy transfer processes induced by collisions of gas-phase species with liquid and semi-solid surfaces.

Gas-surface energy exchange and thermal accommodation of CO2 and Ar in collisions with methyl, hydroxyl, and perfluorinated self-assembled monolayers

Molecular beams of CO(2) and Ar were scattered from long-chain methyl (CH(3)-), hydroxyl (OH-), and perfluoro ((CF(2))(7)CF(3)-) functionalized alkanethiol self-assembled monolayers (SAMs) on gold to study the dynamics of energy exchange and thermal accommodation on model organic surfaces. Ar collisions, for incident energies ranging from 25 to 150 kJ mol(-1), exhibit final energy distributions that depend significantly on the terminal functional group of the SAM. The long-chain CH(3)-terminated monolayers serve as an excellent energy sink for dissipating the incident translational energy. For example, at 150 kJ mol(-1), greater than 90% of the collision energy is transferred to the CH(3)-SAM surface for specularly-scattered atoms (θ(i) = θ(f) = 30° from normal). However, the OH-SAM is a more rigid collision partner due to the formation of an intra-monolayer hydrogen bonding network and the (CF(2))(7)CF(3)-SAM (F-SAM) provides a high degree of rigidity due to the massive CF(3) groups. The final energies for the triatomic, CO(2), scattering from the three surfaces are remarkably similar to the results for Ar scattering. The only significant difference in the translational energy transfer dynamics for these two gases appears in collisions with the OH-SAM. Strong gas-surface attractive forces between CO(2) and the OH-SAM surface appear to counter the rigidity of the hydrogen-bonding network to help bring the majority of the molecules to thermal equilibrium at all incident energies up to 150 kJ mol(-1), resulting in increased energy transfer in comparison to Ar. The similarities in energy transfer for Ar and CO(2) final energy distributions in scattering from the CH(3)- and F-SAMs suggest that the internal degrees of freedom in the triatomic play only a small role in determining the outcome of the gas-surface collision under the scattering conditions employed in this work.

Experimental and theoretical study of CO collisions with CH3- and CF3-terminated self-assembled monolayers

We present an experimental and theoretical study of the dynamics of collisions of the CO molecule with organic surfaces. Experimentally, we scatter CO at 60 kJ mol(-1) and 30 degrees incident angle from regular (CH(3)-terminated) and omega-fluorinated (CF(3)-terminated) alkanethiol self-assembled monolayers (SAMs) and measure the time-of-flight distributions at the specular angle after collision. At a theoretical level, we carry out classical-trajectory simulations of the same scattering process using CO/SAM potential-energy surfaces derived from ab initio calculations. Agreement between measured and calculated final translational energy distributions justifies use of the calculations to examine dynamical behavior of the gas/surface system not available directly from the experiment. Calculated state-to-state energy-transfer properties indicate that the collisions are notably vibrationally adiabatic. Similarly, translational energy transfer from and to CO rotation is relatively weak. These trends are examined as a function of collision energy and incident angle to provide a deeper understanding of the factors governing state-to-state energy transfer in gas/organic-surface collisions.

Boundary Layer Noise from Discrete Roughness Elements

Far field noise was recorded for the flow of a turbulent wall jet boundary layer over various discrete roughness elements including rocks, cubes, and cylinders. The elements were contained within the boundary layer but were a substantial fraction of the boundary layer thickness. Far-field sound measurements were used to infer non-dimensional drag spectra for the elements. The sound radiated from individual elements, from pairs of elements at various streamwise and spanwise separations, and from fetches of elements built progressively from a single element was studied in detail. The coherence between wall pressure fluctuations in the vicinity of a cubic element and the far-field sound radiated by that element was measured. Overall, the results show that that the aerodynamic interaction between elements has remarkably little impact on the sound they generate. Only when the elements are packed as tightly as possible does the total sound produced differ substantially from the sum of that which would be produced by the elements in isolation.

Collisions of sodium atoms with liquid glycerol: Insights into solvation and ionization

The reactive uptake and ionization of sodium atoms in glycerol were investigated by gas-liquid scattering experiments and ab initio molecular dynamics (AIMD) simulations. A nearly effusive beam of Na atoms at 670 K was directed at liquid glycerol in vacuum, and the scattered Na atoms were detected by a rotatable mass spectrometer. The Na velocity and angular distributions imply that all impinging Na atoms that thermally equilibrate on the surface remain behind, likely ionizing to e(-) and Na(+). The reactive uptake of Na atoms into glycerol was determined to be greater than 75%. Complementary AIMD simulations of Na striking a 17-molecule glycerol cluster indicate that the glycerol hydroxyl groups reorient around the Na atom as it makes contact with the cluster and begins to ionize. Although complete ionization did not occur during the 10 ps simulation, distinct correlations among the extent of ionization, separation between Na(+) and e(-), solvent coordination, and binding energies of the Na atom and electron were observed. The combination of experiments and simulations indicates that Na-atom deposition provides a low-energy pathway for generating solvated electrons in the near-interfacial region of protic liquids.

On the accuracy of analytical potentials: comment on ‘Accurate ab initio calculation of the Ar–CF4 intermolecular potential energy surface’

In a recent study of the Ar–CF4 intermolecular interaction potential [Shen C-C, Chang R-Y. Accurate ab initio calculation of the Ar–CF4 intermolecular potential energy surface. Mol Sim. 2010;36:1111–1122], Shen and Chang (SC) illustrated how the use of bond functions can improve the accuracy and basis-set saturation of electronic structure calculations employing perturbation and coupled-cluster theory. SC then used these ab initio data to derive analytic potential energy functions for use in chemical dynamics simulations. We critically examine these analytic potentials and comment on their usage in such simulations. Our analysis highlights the need for care and global validation when deriving analytic potential energy functions.

Particle Beam Scattering From the Vacuum–Liquid Interface

Abstract Knowledge of the molecular-scale collision processes that determine chemical reaction dynamics at liquid surfaces is important for a complete understanding of a range of interfacial processes. The chapter details those research approaches that use directed beams of atoms and molecules to collide with liquid surfaces in vacuum. The chapter begins by discussing ways to satisfy the special requirements presented by conducting research with liquids in a vacuum environment. The contemporary experimental scattering approaches are surveyed, including methods based on time-of-flight mass spectrometry and laser spectroscopy. A discussion of the theoretical approaches to explain molecular scattering behavior ranges from simple kinematic models to fully atomistic direct dynamics simulations.

Performance of a rigid rod statistical mechanical treatment to predict monolayer ordering: a study of chain interactions and comparison with molecular dynamics simulation

A statistical mechanical model that treats hydrocarbon self-assembled monolayer (SAM) chains as rigid rods is examined to interrogate the mechanisms involved in monolayer ordering. The statistical mechanical predictions are compared to fully atomistic molecular dynamics simulations of SAMs with different packing densities. The monolayer chain order is examined as a function of surface coverage, chain-surface interactions, and chain–chain interactions. Reasonable interaction potentials are deduced from ab initio electronic structure calculations of small model systems. It is found that the chain-surface interaction is the most important parameter in formation of flat-lying monolayer phases, while formation of standing phase monolayers is driven most importantly by increased density of molecules at the surface. A brief discussion of the utility and validity of the rigid rod treatment is given in light of the molecular dynamics results.