Dipole moment effects in photodetachment from cluster anions

In this dissertation results from experimental studies on the effect of the dipole moment on photodetachment from solvated iodide anion are presented. The major advantage of using solvated atomic iodide anions: I−·(Y)n: Y = solvent molecule, n=1 to 3)) to study this effect is that in the ground state, the excess electron is mainly localized on the iodine atom for which photodetachment behavior is well understood. The effect of the electric dipole moment of the resultant neutral cluster on the outgoing electron is studied by comparing the photoelectron angular distributions: PADs) for I−·(Y)n and I− photodetachment. The results show strong dipole moment effects in the vicinity of direct detachment thresholds due to the presence of dipole-supported states, [I: 2P3/2)·(Y)n] − and I(2P1/2)·(Y) n]−. Vertical photoexcitation near the threshold for production of the excited neutral cluster I(2P1/2) ·(Y)n shows evidence of strong mixing of the direct: I−·(Y)n→ I: 2P3/2) +: Y)n + e−) channel and a dipole-supported state, [I(2P1/2)·(Y) n]− in the PADs. It is shown that increasing the dipole moment of I(2P1/2)·(Y)n for n=1 increases this channel coupling while for n \u3e 1, the situation is more complex due to competing dynamics. Results are also presented for photodetachment from a stable dipole-bound CH3CN− anion in the vicinity of a well-known electron scattering resonance. Despite the presence of this π *CN it appears to have very little effect on the photoelectron angular distributions. This initially surprising behavior is attributed to the relatively low contribution of higher partial angular momenta waves in the detachment process

Vibronic coupling in the superoxide anion: The vibrational dependence of the photoelectron angular distribution

We present a comprehensive photoelectron imaging study of the O₂(X³Σg⁻,v′=0–6)←O₂⁻(X²Πg,v′′=0) and O₂(a¹Δg,v′=0–4)←O₂⁻(X²Πg,v′′=0)photodetachment bands at wavelengths between 900 and 455 nm, examining the effect of vibronic coupling on the photoelectron angular distribution (PAD). This work extends the v′=1–4 data for detachment into the ground electronic state, presented in a recent communication [R. Mabbs, F. Mbaiwa, J. Wei, M. Van Duzor, S. T. Gibson, S. J. Cavanagh, and B. R. Lewis, Phys. Rev. A82, 011401–R (2010)]. Measured vibronic intensities are compared to Franck–Condon predictions and used as supporting evidence of vibronic coupling. The results are analyzed within the context of the one-electron, zero core contribution (ZCC) model [R. M. Stehman and S. B. Woo, Phys. Rev. A23, 2866 (1981)]. For both bands, the photoelectron anisotropy parameter variation with electron kinetic energy,β(E), displays the characteristics of photodetachment from a d-like orbital, consistent with the π∗g 2p highest occupied molecular orbital of O₂⁻. However, differences exist between the β(E) trends for detachment into different vibrational levels of the X³Σg⁻ and a ¹Δg electronic states of O₂. The ZCC model invokes vibrational channel specific “detachment orbitals” and attributes this behavior to coupling of the electronic and nuclear motion in the parent anion. The spatial extent of the model detachment orbital is dependent on the final state of O₂: the higher the neutral vibrational excitation, the larger the electron binding energy. Although vibronic coupling is ignored in most theoretical treatments of PADs in the direct photodetachment of molecular anions, the present findings clearly show that it can be important. These results represent a benchmark data set for a relatively simple system, upon which to base rigorous tests of more sophisticated models.The authors gratefully acknowledge support by the National Science Foundation Grant No. CHE-0748738 and ANU ARC Discovery Projects under Grant Nos. DP0666267 and DP0880850

Understanding the interactions between triolein and cosolvent binary mixtures using molecular dynamics simulations

Biodiesel is one of the emerging renewable sources of energy to replace fossil-fuel-based resources. It is produced by a transesterification reaction in which a triglyceride reacts with methanol in the presence of a catalyst. The reaction is slow because of the low solubility of methanol in triglycerides, which results in low concentrations of methanol available to react with triglyceride. To speed up the reaction, cosolvents are added to create a single phase which helps to improve the concentration of methanol in the triglyceride phase. In this study, molecular dynamics simulations are used to help understand the role of cosolvents in the solvation of triglyceride (triolein). Six binary mixtures of triolein/cosolvent were used to study the solvation of triolein at 298.15 K. Results of 100 ns simulations at constant temperature and pressure to simulate mixing experiments show that in the first 10 ns all the binary mixtures remain largely unmixed. However, for the cosolvents that are fully miscible with triolein, the partial densities across the simulation boxes show that the systems are fully mixed in the final 10 ns. Some solvents were found to interact strongly with the polar part of triolein, while others interacted with the aliphatic part. The radial distribution functions and clustering of the solvents around triolein were also used as indicators for solvation. The presence of cosolvents also influenced the conformation of triolein molecules. In the presence of solvents that solubilize it, triolein preferred a propeller conformation but took up a trident conformation when there is less or no solubilization. The results show that tetrahydrofuran is the best solvent at solubilizing triolein, followed by cyclopentyl methyl ether, diethyl ether, and hexane. With 1,4-dioxane, the solubility improves with an increase in temperature. The miscibility of a solvent in triolein is aided by its ability to interact with both the polar and nonpolar parts of triolein

Vibronic coupling in the superoxide anion: the vibrational dependence of the photoelectron angular distribution

We present a comprehensive photoelectron imaging study of the O2(X 3Σg−,v′ = 0–6)←O2−(X 2Πg,v″ = 0) and O2(a 1Δg,v′ = 0–4)←O2−(X 2Πg,v″ = 0) photodetachment bands at wavelengths between 900 and 455 nm, examining the effect of vibronic coupling on the photoelectron angular distribution (PAD). This work extends the v′ = 1–4 data for detachment into the ground electronic state, presented in a recent communication. Measured vibronic intensities are compared to Franck–Condon predictions and used as supporting evidence of vibronic coupling. The results are analyzed within the context of the one-electron, zero core contribution (ZCC) model. For both bands, the photoelectron anisotropy parameter variation with electron kinetic energy, β(E), displays the characteristics of photodetachment from a d-like orbital, consistent with the πg∗ 2p highest occupied molecular orbital of O2−. However, differences exist between the β(E) trends for detachment into different vibrational levels of the X 3Σg− and a 1Δg electronic states of O2. The ZCC model invokes vibrational channel specific “detachment orbitals” and attributes this behavior to coupling of the electronic and nuclear motion in the parent anion. The spatial extent of the model detachment orbital is dependent on the final state of O2: the higher the neutral vibrational excitation, the larger the electron binding energy. Although vibronic coupling is ignored in most theoretical treatments of PADs in the direct photodetachment of molecular anions, the present findings clearly show that it can be important. These results represent a benchmark data set for a relatively simple system, upon which to base rigorous tests of more sophisticated models

ReaxFF study of the decarboxylation of methyl palmitate over binary metallic nickel-molybdenum catalysts

Biodiesel has emerged as a possible replacement for fossil-based fuels, particularly in the transportation industry. Because of its high oxygen content, it has several limitations including high viscosity, pour point and cloud point. Converting biodiesel to hydrocarbons is one method of improving the poor flow properties. In this study, Reactive Force Field (ReaxFF) molecular dynamics was used to study the decarboxylation of methyl palmitate on α-NiMoO4, β-NiMoO4 and Ni3Mo catalysts. The results show that the reactions are faster in the presence of α-NiMoO4 and β-NiMoO4, and the number of stable products, carbon dioxide and ethene was higher than they were without the catalyst. With Ni3Mo catalyst, there is rapid initial formation of CO2 and C2H4 until a maximum is reached followed by a decrease in their quantity. The C2H4 was found to decompose to C2H2 and H2 while CO2 was reduced to CO. All reactions were found to follow first-order kinetics, from which the activation energies (Ea) were determined. The Ea drops from 36.89 kcal/mol for uncatalyzed reaction to 25.66, 19.34 and 11.69 kcal/mol for the α-NiMoO4, β-NiMoO4 and Ni3Mo catalysed reactions, respectively. The Ni3Mo catalysed system’s Ea was also closest to the experimentally reported value of 10.11 kcal/mol [1].</p

Observation of vibration-dependent electron anisotropy in O−2 photodetachment

Photoelectron angular distributions (PADs) recorded for the O2(X 3Σg-) ← O2-(X 2Πg) band show significant vibrational dependence. Experimental evidence of vibrational influence on the PAD has, to date, been sparse. Consequently, little attention has