Things we Wrote

	Methyl-cyclohexane methanol (MCHM) isomer-dependent binding on amorphous carbon surfaces View Abstract Abstract In January 2014, over 10,000 gallons of methyl-cyclohexane methanol (MCHM) leaked into the Elk River in West Virginia, in a chemical spill incident that contaminated a large portion of the state’s water supply and left over 300,000 residents without clean water for many days and weeks. Initial efforts to remove MCHM at the treatment plant centered on the use of granulated activated carbon (GAC), which removed some of the chemical from the water, but MCHM levels were not lowered to a “non-detect” status until well after the chemical plume had moved downstream of the intake. Months later, MCHM was again detected at the outflow (but not the inflow) at the water treatment facility, necessitating the full and costly replacement of all GAC in the facility. The purpose of this study is to investigate the hypothesis that preferential absorbance of one of the two MCHM isomers, coupled with seasonal variations in water temperature, explain this contrary observation. Calculated intermolecular potentials between ovalene (a large planar polycyclic aromatic hydrocarbon) and the MCHM isomers were compared to physisorption potentials of MCHM onto an amorphous carbon model. While a molecular mechanics (MM) force field predicts no difference in the average interaction potentials between the cis– and trans-MCHM with the planar ovalene structure, MM predicts that the trans isomer binds stronger than the cis isomer to the amorphous carbon surface. Semi-empirical and density functional theory also predict stronger binding of trans-MCHM on both the planar and amorphous surfaces. The differences in the isomer binding strengths on amorphous carbon imply preferential absorbance of the trans isomer onto activated charcoal filter media. Considering seasonal water temperatures, simple Arrhenius kinetics arguments based on these predicted binding energies help explain the environmental observations of MCHM leeching from the GAC filters months after the spill. Overall, this work shows the important implications that can arise from detailed interfacial chemistry investigations. WA Alexander Molecules 26 (11), 3411	2021
	Sodiumm atom beam collisions with the liquid glycerol surface: Mass effects of deuteration View Abstract Abstract Gas-phase sodium atom collisions with liquid glycerol were investigated by atomic beam scattering from normal and deuterated surfaces. The scattering signal of the recoiling effusive Na atom beam was monitored by a mass spectrometer. Because all thermally equilibrated Na atoms ionize into the liquid surface, this system was uniquely suited to interrogate the subtle kinematic effect of glycerol deuteration on the collisional energy transfer process within the impulsive scattering regime. An interplay of kinematic and vibrational effects explains the Na scattering behavior from variously deuterated glycerol surfaces, extending our fundamental understanding of collisional energy transfer, and ultimately, chemical reaction dynamics. JP Wiens, WA Alexander Chemical Physics Letters 730, 321-325	2019
	Particle beam scattering from the vacuum-liquid Interface View Abstract 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. WA Alexander Ch. 8 in “Physical Chemistry of Gas-Liquid Interfaces”	2018
	Performance of a rigid rod statistical mechanical treatment to predict monolayer ordering: a study of chain interactions and comparison with molecular dynamics simulation View Abstract Abstract 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. WA Alexander Journal of Mathematical Chemistry 55 (2), 423-435	2017
	Enabling Science Support for Better Decision-Making when Responding to Chemical Spills View Abstract Abstract Chemical spills and accidents contaminate the environment and disrupt societies and economies around the globe. In the United States there were approximately 172,000 chemical spills that affected US waterbodies from 2004 to 2014. More than 8000 of these spills involved non–petroleum-related chemicals. Traditional emergency responses or incident command structures (ICSs) that respond to chemical spills require coordinated efforts by predominantly government personnel from multiple disciplines, including disaster management, public health, and environmental protection. However, the requirements of emergency response teams for science support might not be met within the traditional ICS. We describe the US ICS as an example of emergency-response approaches to chemical spills and provide examples in which external scientific support from research personnel benefitted the ICS emergency response, focusing primarily on nonpetroleum chemical spills. We then propose immediate, near-term, and long-term activities to support the response to chemical spills, focusing on nonpetroleum chemical spills. Further, we call for science support for spill prevention and near-term spill-incident response and identify longer-term research needs. The development of a formal mechanism for external science support of ICS from governmental and nongovernmental scientists would benefit rapid responders, advance incident- and crisis-response science, and aid society in coping with and recovering from chemical spills. JL Weidhaas, AM Dietrich, NJ DeYonker, RR Dupont, WT Foreman, D Gallagher, JEG Gallagher, AJ Whelton, WA Alexander Journal of Environmental Quality 45 (5), 1490-1500	2016
	Dipole moments of trans– and cis-(4-methylcyclohexyl)methanol (4-MCHM): obtaining the right conformer for the right reason View Abstract Abstract Accurate computational estimates of fundamental physical properties can be used as inputs in the myriad of extant models employed to predict toxicity, transport, and fate of contaminants. However, as molecular complexity of contaminants increases, it becomes increasingly difficult to determine the magnitude of the errors introduced by ignoring the 3D conformational space averaging within group-additivity and semi-empirical approaches. The importance of considering 3D molecular structure is exemplified for the dipole moments of cis and trans isomers of (4-methylcyclohexyl)methanol (4-MCHM). When 10,000 gallons of 4-MCHM was spilled into the Elk River in January 2014, a lack of toxicological data and environmental partitioning coefficients hindered the immediate protection of human health and the local water supply in West Virginia, USA. Post-spill analysis of the contaminants suggested that the cis and trans isomers had observably different partitioning coefficients and solubility, and thus differing environmental fates. Obtaining high-quality dipole moments using ab initio quantum chemical methods for the isomeric pair was crucial in validating their experimental differences in solubility [Environ. Sci. Technol. Lett., 2015, 2, 127]. The use of first principles electronic structure theory is further explored here to obtain accurate conformer relative energies and dipole moments of cis– and trans-4-MCHM. Overall, the MP2 aug-cc-pVDZ level of theory affords the best balance between accuracy and computational cost. NJ DeYonker, KA Charbonnet, WA Alexander Physical Chemistry Chemical Physics 18 (27), 17856-17867	2016
	On the accuracy of analytical potentials: comment on ‘Accurate ab initio calculation of the Ar–CF₄ intermolecular potential energy surface’ View Abstract Abstract 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. WA Alexander Molecular Simulation 41 (8) 610-612	2015
	Partitioning, Aqueous Solubility, and Dipole Moment Data for cis-and trans-(4-Methylcyclohexyl) methanol, Principal Contaminants of the West Virginia Chemical Spill View Abstract Abstract In 2014, the U.S. National Response Center recorded more than 30000 incidents of oil spills, chemical releases, or maritime security issues, including crude (4-methylcyclohexyl)methanol (MCHM) that contaminated river and drinking water in West Virginia. This research yielded physicochemical partitioning data for the two major compounds released in West Virginia, cis– and trans-(4-methylcyclohexyl)methanol. Octanol–water partition coefficients (KOW) were 225 for cis-4-MCHM and 291 for trans-4-MCHM. The aqueous solubility for total 4-MCHM was 2250 mg/L at 23 °C; solubilities of individual isomers were dependent on their mole fractions. The cis isomer was more soluble and less well sorbed to activated carbon than the trans isomer, consistent with its lower KOW. The partition behavior is supported by a larger computed solvated dipole moment for the cis form than for the trans form at the MP2 aug-cc-pwCVDZ SMD level of theory. Different partition properties would result in the differential fate and transport of cis– and trans-4-MCHM in aqueous environments. AM Dietrich, A Thomas, Y Zhao, E Smiley, N Shanaiah, M Ahart, KA Charbonnet, NJ DeYonker, WA Alexander, DL Gallagher Environmental Science & Technology Letters 2 (4), 123-127	2015
	Collisions of Sodium Atoms with Liquid Glycerol: Insights into Solvation and Ionization View Abstract Abstract 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. JP Wiens, GM Nathanson, WA Alexander, TK Minton, S Lakshmi, GC Schatz Journal of the American Chemical Society 136 (8), 3065-3074	2014
	Interfacial energy exchange and reaction dynamics in collisions of gases on model organic surfaces View Abstract Abstract Molecular beam scattering experiments and molecular dynamics simulations have been combined to develop an atomic-level understanding of energy transfer, accommodation, and reactions during collisions between gases and model organic surfaces. The work highlighted in this progress report has been motivated by the scientific importance of understanding fundamental interfacial chemical reactions and the relevance of reactions on organic surfaces to many areas of environmental chemistry. The experimental investigations have been accomplished by molecular beam scattering from ω-functionalized self-assembled monolayers (SAMs) on gold. Molecular beams provide a source of reactant molecules with precisely characterized collision energy and flux; SAMs afford control over the order, structure, and chemical nature of the surface. The details of molecular motion that affect energy exchange and scattering have been elucidated through classical-trajectory simulations of the experimental data using potential energy surfaces derived from ab initio calculations. Our investigations began by employing rare-gas scattering to explore how alkanethiol chain length and packing density, terminal group relative mass, orientation, and chemical functionality influence energy transfer and accommodation at organic surfaces. Subsequent studies of small molecule scattering dynamics provided insight into the influence of internal energy, molecular orientation, and gas–surface attractive forces in interfacial energy exchange. Building on the understanding of scattering dynamics in non-reactive systems, our work has recently explored the reaction probabilities and mechanisms for O3 and atomic fluorine in collisions with a variety of functionalized SAM surfaces. Together, this body of work has helped construct a more comprehensive understanding of reaction dynamics at organic surfaces. JW Lu, BS Day, LR Fiegland, ED Davis, WA Alexander, D Troya, JR Morris Progress in Surface Science 87 (9), 221-252	2012
	Reactions of Solvated Electrons Initiated by Sodium Atom Ionization at the Vacuum-Liquid Interface View Abstract Abstract Solvated electrons are powerful reagents in the liquid phase that break chemical bonds and thereby create additional reactive species, including hydrogen atoms. We explored the distinct chemistry that ensues when electrons are liberated near the liquid surface rather than within the bulk. Specifically, we detected the products resulting from exposure of liquid glycerol to a beam of sodium atoms. The Na atoms ionized in the surface region, generating electrons that reacted with deuterated glycerol, C₃D₅(OD)₃, to produce D atoms, D₂, D₂O, and glycerol fragments. Surprisingly, 43 ± 4% of the D atoms traversed the interfacial region and desorbed into vacuum before attacking C-D bonds to produce D₂. WA Alexander, JP Wiens, TK Minton, GM Nathanson Science 335 (6072), 1072-1075	2012
	Kinematics and dynamics of atomic-beam scattering on liquid and self-assembled monolayer surfaces View Abstract Abstract 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. WA Alexander, J Zhang, VJ Murray, GM Nathanson, TK Minton Faraday discussions 157, 355-374	2012
	Initial Reaction Probability and Dynamics of Ozone Collisions with a Vinyl-Terminated Self-Assembled Monolayer View Abstract Abstract The gas–surface reaction dynamics of ozone with a model unsaturated organic surface have been explored through a series of molecular beam scattering experiments. Well-characterized organic surfaces were reproducibly created by adsorption of C═C-terminated long-chain alkanethiols onto gold, while the incident molecular beams were created by supersonic expansion of ozone seeded in an inert carrier gas to afford control over collision energy. Time-of-flight distributions for the scattered molecules showed near complete thermal accommodation of ozone for incident energies as high as 70 kJ/mol. Reflection–absorption infrared spectroscopy, performed in situ with ozone exposure, revealed that oxidation of the double bond depends significantly on the translational energy of O3. For energies near room temperature, 5 kJ/mol, the initial reaction probability (γ0) for the formation of the primary ozonide was determined to be γ₀ = 1.1 × 10^–5. As translational energy increased to 20 kJ/mol, the reaction probability decreased. This behavior, along with a strong inverse relationship between γ0 and surface temperature, demonstrates that the room-temperature reaction follows the Langmuir–Hinshelwood mechanism, requiring accommodation prior to reaction under nearly all atmospherically relevant conditions. However, measurements show that the dynamics transition to a direct reaction (analogous to the Eley–Rideal mechanism) for elevated translational energies. JW Lu, LR Fiegland, ED Davis, WA Alexander, A Wagner, RD Gandour, JR Morris The Journal of Physical Chemistry C 115 (51), 25343-25350	2011
	Theoretical Study of the Dynamics of Collisions Between HCl and ω-Hydroxylated Alkanethiol Self-Assembled Monolayers View Abstract Abstract We present a classical-trajectory study of collisions of HCl with hydroxylated alkanethiol self-assembled monolayers. The potential-energy surface used in the trajectory propagation is a combination of the standard OPLS force field to describe the surface and an analytical potential for the gas/surface interaction developed in this work. The gas/surface potential has been derived based on high-quality electronic-structure calculations of model HCl−alcohol systems in the gas phase and includes a flexible Buckingham term and a Coulombic term. The results of the trajectories calculations are in good agreement with recent molecular-beam experiments on the same system, thereby lending support to the accuracy of the calculations. The collision dynamics differ vastly from prior scattering studies involving rare gases and CO, primarily because the gas/surface attraction governed by hydrogen bonding dramatically increases the ability of the gas molecules to trap on the surface for extended times. The properties of the desorbing HCl molecules are largely insensitive to the initial collision energy and are only mildly affected by the incident angle. An analysis of the reaction mechanism reveals the distinct dynamics of trajectories that either recoil from the surface directly or undergo multiple collisions with the surface and result in thermalization. WA Alexander, D Troya The Journal of Physical Chemistry C 115 (5), 2273-2283	2011
	Gas–surface energy exchange and thermal accommodation of CO₂ and Ar in collisions with methyl, hydroxyl, and perfluorinated self-assembled monolayers JW Lu, WA Alexander, JR Morris Physical Chemistry Chemical Physics 12 (39), 12533-12543	2010
	Experimental and theoretical study of CO collisions with CH₃-and CF₃-terminated self-assembled monolayers View Abstract Abstract 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 and 30° incident angle from regular (CH₃-terminated) and ωω-fluorinated (CF₃-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. WA Alexander, JR Morris, D Troya The Journal of chemical physics 130 (8), 084702	2009
	Theoretical Study of the Stereodynamics of CO Collisions with CH₃-and CF₃-Terminated Alkanethiolate Self-Assembled Monolayers† View Abstract Abstract We present a classical-trajectory study of CO collisions with regular (CH₃-terminated) and ω-fluorinated (CF₃-terminated) alkanethiol self-assembled monolayers (SAMs) with a focus on analyzing the stereodynamics properties of the collision. The CO molecule is scattered with incident angles of either 30° or 60° with respect to the surface normal and with 60 kJ/mol collision energy, and we analyze final translational and rotational energy, mechanism of the collisions, and orientation and alignment of the rotational angular momentum. Analysis of the alignment of the final rotational angular momentum in collisions involving initially rotationally cold CO indicates a slight preference for “cartwheel” and “corkscrew” rotational motions. In contrast, collisions of initially excited CO slightly favor “helicopter” motion of the recoiling molecule. Moreover, studies of final orientation reveal that, while cartwheel “topspin” motion is favored for collisions in which initially cold CO becomes rotationally excited, no preferred handedness is observed when CO leaves the surfaces with “helicopter” motion. Analysis of trajectories involving initially rotationally excited CO in which the initial rotational angular momentum is aligned and/or oriented shows a non-negligible effect of the initial rotational motion on the dynamics of energy transfer. For instance, CO approaching the SAMs with helicopter motion retains a larger fraction of its initial rotation than molecules colliding with cartwheel-type motions. Conservation of the alignment and orientation of the initial rotational angular momentum vector is also enhanced with helicopter motion relative to cartwheel or random motions. The calculated trends in the stereodynamic properties for the two SAMs indicate that the CH₃-SAM is effectively more corrugated than the CF₃-SAM. WA Alexander, JR Morris, D Troya The Journal of Physical Chemistry A 113 (16), 4155-4167	2009
	Collisions of Polar and Nonpolar Gases with Hydrogen Bonding and Hydrocarbon Self-Assembled Monolayers View Abstract Abstract Molecular beam scattering experiments are used to explore collisions of 60 kJ/mol Ne, CD₄, ND₃, and D₂O with long-chain CH₃-, NH₂-, and OH-terminated self-assembled monolayers (SAMs) created via the chemisorption of alkanethiols on gold. Time-of-flight measurements for the scattered gases reveal the extent of energy exchange and the propensity for a gas to thermally accommodate with the surface during a collision. Of the four gases studied, Ne transfers the least amount of translational energy into the monolayers and D₂O the most. Neon atoms recoil from the OH-SAM with an average of 14.4 kJ/mol of energy, while D₂O retains only 6.4 kJ/mol of its 60 kJ/mol incident energy when scattering from the same surface. Overall, the trend in final translational energies follows the order Ne > CD₄ > ND₃ > D₂O for scattering from all three SAMs. The observed trend in the energy exchange is correlated with the gas−surface attractive forces, as determined by ab initio calculations. The thermal accommodation efficiencies of the four gases follow the opposite trend. Thermalization for the Ne atoms is nearly negligible for all three monolayers, whereas D₂O and ND3 approach near complete accommodation on all of the monolayers studied. The overall energy exchange and thermal accommodation efficiencies also depend markedly on the terminal group of the SAM. For Ne scattering, the trend for the overall energy transfer follows: CH₃– > NH₂– ≈ OH-SAMs. In contrast, the overall D₂O energy transfer is greater when colliding with the OH-SAM than the nonpolar CH₃-SAM. Together, the results show that the extent of energy transfer depends on a balance between the rigidity of the surface, as affected by intrasurface hydrogen bonding, and the strength of the gas−surface attractive forces, as determined by intermolecular interactions. ME Bennett, WA Alexander, JW Lu, D Troya, JR Morris The Journal of Physical Chemistry C 112 (44), 17272-17280	2008
	Experimental and theoretical studies of the effect of mass on the dynamics of gas/organic-surface energy transfer View Abstract Abstract The effect of mass on gas/organic-surface energy transfer is explored via investigation of the scattering dynamics of rare gases (Ne, Ar, and Kr) from regular (CH₃-terminated) and ω-fluorinated (CF₃-terminated) alkanethiol self-assembled monolayers (SAMs) at 60 kJ/mol collision energy. Molecular-beam scattering experiments carried out in ultrahigh vacuum and molecular-dynamics simulations based on high-accuracy potentials are used to obtain the rare-gases’ translational-energy distributions after collision with the SAMs. Simulations indicate that mass is the most important factor in determining the changes in the energy exchange dynamics for Ne, Ar, and Kr collisions on CH₃– and CF₃-terminated SAMs at 60 kJ/mol collision energy. Other factors, such as changes in the gas-surface potential and intrasurface interactions, play only a minor role in determining the differential dynamics behavior for the systems studied. WA Alexander, BS Day, HJ Moore, TR Lee, JR Morris, D Troya Journal of Chemical Physics 128 (1), 14713-14713	2008
	Theoretical study of the Ar-, Kr-, and Xe-CH₄,-CF₄ intermolecular potential-energy surfaces View Abstract Abstract We present a theoretical study of the intermolecular potentials for the Ar, Kr, and Xe−CH₄, −CF₄ systems. The potential-energy surfaces of these systems have been calculated utilizing second-order Möller−Plesset perturbation theory and coupled-cluster theory in combination with correlation-consistent basis sets (aug-cc-pvnz; n = d, t, q). The calculations show that the stabilizing interactions between the rare gases and the molecules are slightly larger for CF₄ than for CH₄. Moreover, the rare-gas−CX₄ (X = H, F) potentials are more attractive for Xe than for Kr and Ar. Our highest quality ab initio data (focal-point-CCSD(T) extrapolated to the complete basis set limit) have been used to develop pairwise analytical potentials for rare-gas−hydrocarbon (−fluorocarbon) systems. These potentials can be applied in classical-trajectory studies of rare gases interacting with hydrocarbon surfaces. WA Alexander, D Troya The Journal of Physical Chemistry A 110 (37), 10834-10843	2006
	Theoretical study of the effect of surface density on the dynamics of Ar+ alkanethiolate self-assembled monolayer collisions View Abstract Abstract We present a classical-trajectory study of energy transfer in collisions of Ar atoms with alkanethiolate self-assembled monolayers (SAMs) of different densities. The density of the SAMs is varied by changing the distance between the alkanethiolate chains in the organic monolayers. Our calculations indicate that SAMs with smaller packing densities absorb more energy from the impinging Ar atoms, in agreement with recent molecular-beam scattering experiments. We find that energy transfer is enhanced by a decrease in the SAM density because (1) less dense SAMs increase the probability of multiple encounters between Ar and the SAM, (2) the vibrational frequencies of large-amplitude motions of the SAM chains decrease for less dense SAMs, which makes energy transfer more efficient in single-encounter collisions, and (3) increases in the distance between chains promote surface penetration of the Ar atom. Analysis of angular distributions reveals that the polar-angle distributions do not have a cosine shape in trapping−desorption processes involving penetration of the Ar atom into the alkanethiolate self-assembled monolayers. Instead, there is a preference for Ar atoms that penetrate the surface to desorb along the chain-tilt direction. BS Day, JR Morris, WA Alexander, D Troya The Journal of Physical Chemistry A 110 (4), 1319-1326	2006

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