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Gas Phase Chemical Physics Program
Stephen R. Leone
Daniel M. Neumark
Kevin R. Wilson
The GPCP program at LBNL seeks to expand the fundamental science base that underlies current and future problems in energy sciences, which center on chemical conversion processes, associated with energy production and the coupling between such processes and the environment. The program is not focused on device-level technologies but rather basic science, with the expectation that any new technological advances are enabled by the long term commitment to uncovering the fundamental principles that control chemical reactivity in ground and excited of molecules in the gas phase, in more complex environments, such as clusters, and nanoparticle surfaces. The program supports Department of Energy’s Basic Energy Sciences mission to develop, “…a fundamental understanding of chemical reactivity enabling validated theories, models and computational tools for predicting rates, products, and dynamics of chemical processes involved in energy utilization, and its environmental impacts.”
To address its objectives, LBNL’s GPCP program is organized into three Subtasks:
Subtask 1. Fundamental Molecular Spectroscopy and Chemical Dynamics
Subtask 2. Molecular Reactivity in Complex Systems
Subtask 3. Theory of Electronic Structure and Chemical Dynamics
Organizational structure of the LBNL Gas Phase Chemical Physics Program (GPCP) showing its three integrated Subtasks, as well as the three main experimental and computational BES user facilities that it benefits from.
The three Subtasks provide a sound framework to support the overall goals of the program. Since the making of new bonds, and the breaking of existing bonds occurs at the level of interaction between a pair of atoms, exploring and understanding chemical reactivity is reductionist science. Therefore, as shown in the diagram below, Subtask 1 begins at the level of unimolecular and bimolecular reactive processes, which are the building blocks of kinetic networks. Such building blocks are then embedded in problems of greater complexity in Subtask 2, where their effect on cluster or particle growth, or in multi-phase environments can be interrogated. Subtask 1 is built around the appropriate experimental techniques to understand links between energy flow leading from photon absorption to products, ranging from established molecular beam experiments to new core spectroscopies. Interpreting these experiments, including new probes of core spectroscopy being pioneered in Subtask 1 leads to strong connections with the theory investigators, who will be active collaborators in both Subtask 1 and Subtask 2. Last but not least, in Subtask 3, new electronic structure tools are developed, which are deployed in many of the chemical studies of Subtasks 1 and 2. As a natural consequence of this integrated structure, each PI contributes to efforts in multiple Subtasks. The program makes extensive use of BES-supported user facilities such as the Advanced Light Source (ALS), the Molecular Foundry and the National Energy Research Scientific Computing Center.
Below we summarize the objectives of each Subtask and present a representative project.
Subtask 1: Fundamental Molecular Spectroscopy and Chemical Dynamics
Martin Head-Gordon, Stephen Leone, Daniel M. Neumark, Eric Neuscamman, and David G. Prendergast
The planned research in this Subtask aims to improve our fundamental understanding of unimolecular and bimolecular reactions, anchored by measurements of radical spectroscopy and excited state dynamics. Such measurements employ high-resolution cold anion photoelectron spectroscopy, radical beam photodissociation product coincidence spectroscopy, and flash pyrolysis laser photodissociation spectroscopy, which were pioneered in this program. The very recent development of femtosecond time-resolved table-top X-ray spectroscopic probing of chemical reactions at sufficiently high photon energies to access the carbon K-edge will be extended into the attosecond regime. Photoexcited chemical systems ranging from oxyperoxy radicals, to polycyclic aromatic hydrocarbon radicals to a variety of organic heterocycles will be studied, to characterize their photodissociation dynamics. With embedded computational collaborations, this Subtask advances understanding of dynamics and energetics of reactive species on multiple fronts, providing insights that inform the experiments of Subtask 2, as well as challenges for modeling and simulation that inform as well as benefit from Subtask 3.
Toluene decomposition products whose photodissociation has been investigated with XBeam.
Subtask 2: Molecular Reactivity in Complex Systems
Musahid Ahmed, Martin Head-Gordon, Daniel Neumark, David G. Prendergast, and Kevin R. Wilson
Efforts in Subtask 2 target more complex chemical transformations that extend beyond the gas phase reactivity addressed in Subtask 1. The approach, conceptually, is to build complexity from isolated elementary bimolecular reactions to gas surface reaction dynamics to coupled networks of elementary unimolecular and bimolecular pathways in multiphase systems. Experimental plans begin with growth processes in hydrocarbon free radical reactions (e.g. towards graphene-like and fullerene-like structures) and photoionization dynamics in van der Waals and hydrogen bonded clusters, probed using supersonic jets that will inform how non-covalent interactions such as hydrogen bonding, electrostatics, and van der Waals forces affect formation, destruction and dynamics. In a new direction in gas surface reactions, elastic, inelastic and most importantly reactive scattering of atoms and free radicals from the liquid interfaces of flat jets will be experimentally studied.
(A) Potential energy surface of CHD and (B) schematic orbital diagrams showing evolution of the electronic structure.
The role of the bulk solvent and the surface should affect the dynamics substantially compared to binary interactions that govern gas phase collision dynamics. This approach feeds into more complicated multiphase reaction pathways at organic nanoparticle surfaces, to understand chemical reactivity of key species such as peroxy and alkoxy radicals and Criegee intermediates. In each area of focus, experimental probes are complemented by computational simulations, spanning valence and core electronic structure, and reaction dynamics.
Schematic of flat jet incorporated into crossed beams scattering chamber with electron impact ionization.
Subtask 3: Theory of Electronic Structure and Chemical Dynamics
Martin Head-Gordon, Eric Neuscamman, and David Prendergast
This Subtask is concerned with cross-cutting developments in theoretical methods that complement the specific systems and experiments of interest in Subtasks 1 and 2. The proposed research aims to develop electronic structure tools of broad value. Examples include plans for high accuracy double hybrid density functionals, and advances in wavefunction-based Quantum Monte Carlo approaches for excited states. These branches of development offer complementary domains of applicability: DFT requires far lower computational cost, but also yields lower accuracy than the QMC approaches. The latter are particularly appealing for problems where present-day density functionals fail. The theorists interact on the assessment of their methods, and on complementary approaches to problems of common interest (also to Subtasks 1 and 2) such as valence and core-excited states. The results of this research will, as they mature, yield benefits for chemical applications not only in Subtasks 1 and 2, but also across other areas of modern chemical science including condensed phase modeling, catalysis and energy conversion.
First five singlet excitations in C2 as evaluated by traditional quantum chemistry (CIS and EOM-CCSD), a high-level multi-reference benchmark (MRCI+Q), and our small multi-Slater Jastrow expansions (MSJ). Asterisks mark double excitations. The basis set is cc-pVTZ.