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NR 20140805

An atomistic control of chemical bonds formation and cleavage holds the key to making molecular transformations more energy efficient and product selective. However, inherent scaling relations among binding strengths of adsorbates on various catalytic materials often give rise to volcano-shaped relationships between the catalytic activity and the affinity of critical intermediates to the surface. The optimal catalysts should bind the reactants 'just right', i.e., neither too strong nor too weak, which is the Sabatier's principle. It is extremely useful for searching promising catalysts, but also imposes serious constraints on design flexibility. Therefore, how to circumvent scaling constraints is crucial for advancing catalytic science. It has been shown that hot electrons can selectively activate the chemical bonds that are not responsive to phonon excitation, thus providing a rational approach beyond scaling limitation. Another emerging yet effective way to break the scaling constraint is single atom catalysis. Strong interactions of supported single atoms with supports dramatically affect the electronic structure of active sites, which reroutes mechanistic pathways of surface reactions. In my PhD research, we use CO oxidation reaction on metal-based active sites as a benchmark system to tailor mechanistic pathways through those two strategies 1) ultra-fast laser induced nonadiabatic surface chemistry and 2) oxide-supported single metal catalysis, with the aim to go beyond the Sabatier activity volcano in metal catalysis. / Doctor of Philosophy / Catalysis is the process of increasing the chemical reaction rate by lowering down the activation barrier. There are three different types of catalysis including enzyme, homogeneous, and heterogeneous catalysis. Heterogeneous catalytic reactions involve a sequence of elementary steps, e.g., adsorption of reactants onto the solid surface, transformation of adsorbed species, and desorption of the products. However, the existing scaling relations among binding energies of reaction intermediates on various catalytic materials lead to volcano-shaped relationships, which show the reaction activity versus the binding energy of critical intermediates. The optimal catalysts should bind the reaction intermediates neither too strong nor too weak. This is the Sabatier's principle, which provides useful guidance for searching promising catalysts. But it also imposes the constraint on the attainable catalytic performance. How to break the constraint to further improve the catalytic activity is an emerging problem. The recent studies have shown that the hot surface electrons on the metal surfaces induced by the ultra-fast laser can selectively activate the chemical bonds, thus providing a rational approach beyond scaling constraints. Another way to break the scaling constraint is single atom catalysis. The metal oxides are frequently used as the support to stabilize the single metal atoms. The strong interaction between the single metal atoms and the support affects the electronic structure of the catalysts. Thereby catalytic reactions on the single metal atoms catalyst are very different from that on metal surfaces. In my PhD research, we use CO oxidation reaction as a benchmark system, to tailor reaction pathways through those two strategies on 1) Ru(0001) under ultra-fast laser pulse and 2) Ir single metal atoms supported on spinel oxides, to go beyond Sabatier activity volcano in metal catalysis.

Nonradiative decay of excitons is a competing process to Multi-Exciton Generation (MEG) in nanoparticles. Nonradiative decay of single excitons with sufficient energy to generate bi-excitons in Cd₂₀ Se₁₉ and Cd₈₃ Se₈₁ nanoparticles was studied using Tully's Molecular Dynamics with Quantum Transitions (MDQT) method and a CdSe pseudopo- tential. Exciton decay rates increase with increases in nanoparticle temperature and density of lower-lying excitonic states. There did not appear a significant effect of size on energy decay rates. The decay dynamics generally follow a gradual decay with transitions between nearby states. This is punctuated by periodic, short-lived periods of rapid downhill tran- sitions that result in a large proportion of excess exciton energy being transferred to the vibrational motion of the nanoparticle. The time for relaxation to below the 2.0E[subscript g] cutoff was on the order of 1ps. / text

Nonadiabatic dynamical processes are ubiquitous in chemistry and biology. Such events are directly connected to the treatment of energetically close lying states which gives rise to strong vibronic interactions in which case the Born-Oppenheimer approximation tends to break down. In case of biradicals, nonadiabatic events are facilitated by conical intersections, as a result of symmetry lowering of degenerate electronic states due to Jahn-Teller distortion. A central problem in the treatment of the nonadiabatic molecular dynamics is posed by the representation of potential energy surfaces. A point by point calculation of a potential energy surface on a multi-dimensional grid is very cumbersome and in general does not provide with an analytical functional form of the potential. This becomes even more complicated when the adiabatic surfaces have cusps, where the function becomes non-differentiable. Vibronic model Hamiltonians, which represent the potential in the form of a potential matrix which contains the electronic energies as well as the couplings in a diabatic basis. A Taylor series expansion of the potential matrix can be done to get a smooth analytical functional form of the potential matrix elements. These models can then be used to perform nuclear dynamics using either exact diagonalization time-independent method or the wavepacket propagation based time-dependent methods. Thus, vibronic models provide a compact representation of complicated coupled potential energy surfaces, which can be used in conjunction with non-adiabatic nuclear dynamics Vibronic models have been constructed for selected biradicals, for which photodetachment spectra have been simulated using the time-independent (VIBRON) as well as time-dependent (MCTDH) methods. Consistent results have been obtained with both the approaches for small systems. This also assures the use of MCTDH program for larger systems, where the time-independent methods are not applicable. Moreover, for biradicals, the parent anionic state also undergoes a Jahn-Teller distortion, or often the ground state potential energy surface is highly anharmonic in nature. This requires the description of anionic ground state by a vibronic model. Therefore, in order to simulate the photodetachment spectra of biradicals, three vibronic models are constructed for each simulation. The first model describes the ground and excited states of the parent anionic (neutral) species. Two other vibronic models describe singlet and triplet states of the target neutral (cation) species, and the spectrum is simulated using the vibronic ground state(s) of the anion (neutral) as the absorbing state in VIBRON/MCTDH. The electronic states and vibronic model parameters are obtained using the IP-EOM-CCSD and DIP-STEOM-CCSD methodology as coded in the ACESII quantum chemistry program package. The photodetachment spectra of nitrate radical, cyclobutadiene negative ion and trimethylene negative ion have been studied using this methodology.

Nonadiabatic dynamical processes are ubiquitous in chemistry and biology. Such events are directly connected to the treatment of energetically close lying states which gives rise to strong vibronic interactions in which case the Born-Oppenheimer approximation tends to break down. In case of biradicals, nonadiabatic events are facilitated by conical intersections, as a result of symmetry lowering of degenerate electronic states due to Jahn-Teller distortion. A central problem in the treatment of the nonadiabatic molecular dynamics is posed by the representation of potential energy surfaces. A point by point calculation of a potential energy surface on a multi-dimensional grid is very cumbersome and in general does not provide with an analytical functional form of the potential. This becomes even more complicated when the adiabatic surfaces have cusps, where the function becomes non-differentiable. Vibronic model Hamiltonians, which represent the potential in the form of a potential matrix which contains the electronic energies as well as the couplings in a diabatic basis. A Taylor series expansion of the potential matrix can be done to get a smooth analytical functional form of the potential matrix elements. These models can then be used to perform nuclear dynamics using either exact diagonalization time-independent method or the wavepacket propagation based time-dependent methods. Thus, vibronic models provide a compact representation of complicated coupled potential energy surfaces, which can be used in conjunction with non-adiabatic nuclear dynamics Vibronic models have been constructed for selected biradicals, for which photodetachment spectra have been simulated using the time-independent (VIBRON) as well as time-dependent (MCTDH) methods. Consistent results have been obtained with both the approaches for small systems. This also assures the use of MCTDH program for larger systems, where the time-independent methods are not applicable. Moreover, for biradicals, the parent anionic state also undergoes a Jahn-Teller distortion, or often the ground state potential energy surface is highly anharmonic in nature. This requires the description of anionic ground state by a vibronic model. Therefore, in order to simulate the photodetachment spectra of biradicals, three vibronic models are constructed for each simulation. The first model describes the ground and excited states of the parent anionic (neutral) species. Two other vibronic models describe singlet and triplet states of the target neutral (cation) species, and the spectrum is simulated using the vibronic ground state(s) of the anion (neutral) as the absorbing state in VIBRON/MCTDH. The electronic states and vibronic model parameters are obtained using the IP-EOM-CCSD and DIP-STEOM-CCSD methodology as coded in the ACESII quantum chemistry program package. The photodetachment spectra of nitrate radical, cyclobutadiene negative ion and trimethylene negative ion have been studied using this methodology.

Particles trapped in the magnetosphere are naturally accelerated by the exchange of electromagnetic and kinetic energy, resulting in relativistic plasma populations. Through a number of processes, these particles can be scattered into the atmosphere and lost to interactions. Such precipitating particles can affect radio communications, ozone chemistry, and thermal structures. For these reasons, it is important to characterize loss mechanisms and quantify precipitation rates. This thesis examines one particular loss mechanism known as current sheet scattering (CSS). If interactions are negligible, charged particles in a magnetic field have approximately conserved quantities that characterize their motion provided the background field changes sufficiently slowly over space and time. The first of these ‘adiabatic invariants,’ the magnetic moment, is related to the particle’s mirror point along its bounce trajectory—the location at which the particle reverses direction in its journey from weaker to stronger B. In the equatorial region of the near-Earth magnetotail, where the radius of field line curvature of the magnetic field can become comparable to the gyroradius of ≈ 100 keV electrons, the homogeneity conditions needed for conservation of the magnetic moment of this population are broken. Upon passing through this location, known as the current sheet, these particles experience a chaotic change in their magnetic moment, and thus an alteration of their mirror point. This is the phenomenon of CSS. If the resulting mirror point lies within the atmosphere, the particle will most likely be lost through interactions. CSS is often investigated for highly relativistic electrons. However, recent observations suggest that this mechanism may account for a significant proportion of precipitating electrons between 100 and 300 keV during the substorm growth phase, a common space weather event wherein magnetic field lines in the near-Earth magnetotail become highly stretched. In this thesis, we test the efficacy of CSS as a loss mechanism for < 300 keV electrons by developing a relativistic charged particle tracer capable of solving complex trajectories in realistic magnetospheric magnetic field models. We then find distributional characteristics through Monte Carlo methods, comparing simulated ratios of loss- to total-flux with observations of the same quantities for a single substorm event. These observations are obtained by comparison of in situ measurements made by THEMIS (Time History of Events and Macroscale Interactions during Substorms) with ionospheric energy flux remotely sensed by PFISR (Poker Flat Incoherent Scatter Radar). Given an input distribution from THEMIS satellite measurements, we find agreement between observed and simulated loss- to total-flux ratios within an order of magnitude, with closer agreement for electrons between 100 and 300 keV. This implies CSS can explain a significant proportion of observed precipitation for the event studied and demonstrates its role as a prominent radiation belt loss mechanism. In particular, these findings suggest that the measured loss flux of < 300 keV electrons during such events can be immediately related to the geometry of the near-Earth magnetotail. This is further supported by a parametric study of initially field aligned distributions spawned at various nightside locations, showing a low-energy peak in the loss- to total-flux ratio at the boundary between the outermost radiation belt and the magnetotail. Measurements of particle orientation taken from THEMIS are low resolution, and agreement between simulated and observed loss- to total-flux ratios can be increased by assuming a more field aligned distribution for electrons below 100 keV. This suggests the presence of other physical processes besides CSS that may preferentially structure the pitch angle distributions of low energy electrons to be field aligned. Additional analysis is needed to identify these possible mechanisms. In summary, findings from this work support the role of CSS as an important contributor to < 300 keV electron loss during the substorm growth phase. Though there is an underestimation of loss for < 100 keV electrons, it is known that the empirical magnetic field models employed overestimate the radius of curvature in the current sheet. Furthermore, the dawn-dusk electric field has been neglected, though it has the possibility to produce field aligned electrons through current sheet acceleration. The inclusion of these effects in future studies may further improve agreement between simulation and observations.

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In this thesis quantum-classical dynamics is applied to the study of quantum condensed phase processes. This approach is based on the quantum-classical Liouville equation where the dynamics of a small subset of the degrees of freedom are treated quantum mechanically while the remaining degrees of freedom are treated by classical mechanics to a good approximation. We use this approach as it is computationally tractable, and the resulting equation of motion accurately accounts for the quantum and classical dynamics, as well as the coupling between these two components of the system. By recasting the quantum-classical Liouville equation into the form of a generalized master equation we investigate connections to surface-hopping. The link between these approaches is decoherence arising from interaction of the subsystem with the environment. We derive an evolution equation for the subsystem which contains terms accounting for the effects of the environment. One of these terms involves a memory kernel that accounts for the coherent dynamics. If this term decays rapidly, a Markovian approximation can be made. By lifting the resulting subsystem master equation into the full phase space, we obtain a Markovian master equation that prescribes surface-hopping-like dynamics. Our analysis outlines the conditions under which such a description is valid. Next, we consider the calculation of the rate constant for a quantum mechanical barrier crossing process. Starting from the reactive-flux autocorrelation function, we derive a quantum-classical expression for the rate kernel. This expression involves quantum-classical evolution of a species operator averaged over the initial quantum equilibrium structure of the system making it possible to compute the rate constant via computer simulation. Using a simple model for a proton transfer reaction we compare the results of the rate calculation obtained by quantum-classical Liouville dynamics with that of master equation dynamics. The master equation provides a good approximation to the full quantum-classical Liouville calculation for our model and a more stable algorithm results due to the elimination of oscillating phase factors in the simulation. Finally, we make use of the theoretical framework established in this thesis to analyze some aspects of decoherence used in popular surface-hopping techniques.

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