Benjamin Levine - US grants

In this project funded by the Chemical Theory, Models, and Computational Methods program, Professor Benjamin G. Levine of Michigan State University is developing new computational methods to model non-radiative recombination in semiconductor nanomaterials. Non-radiative recombination is a fundamental process that converts electronic energy to heat, thus limiting the efficiencies of devices for solar energy conversion, light emission, and other applications. The methods being developed provide an atom-by-atom, electron-by-electron mechanistic picture of this process, thus informing the design of future materials. Specifically, the Levine group is applying these new theoretical models to elucidate the dynamics of electronically excited silicon nanoparticles, which show promise as light emitters for solid state displays, biological imaging, and solid state lasers. The Levine group is also working with local high school teachers to develop the High School Computational Chemistry Server (HiSoCCS), which freely serves research-grade computational chemistry capability via a user-friendly web-based interface. HiSoCCS and associated curricular materials are made available for free to Michigan high schools.

The Levine group's approach is based on the hypothesis that conical intersections, points of degeneracy between electronic states, are introduced by defects in the material, and that these intersections provide efficient pathways for non-radiative recombination. A new fully quantum mechanical method for modeling dynamics near conical intersections, the diabatized Gaussians on adiabatic surfaces (DGAS) approximation, is being developed. The DGAS wave function ansatz is designed to handle singularities in the first- and second-derivative nonadiabatic couplings that occur at conical intersections. The DGAS method is being implemented for high performance parallel computers, and the resulting software will be made freely available to other scientists. The broader impacts of this work include: a) the development of a new nonadiabatic molecular dynamics method capable of accurately modeling a wide range non-radiative processes, b) new open source software for modeling dynamics near conical intersections, c) a deeper understanding of non-radiative recombination in silicon nanomaterials, and d) new tools and materials that incorporate research grade calculations into the high school science curriculum.

One of the challenges in chemistry is to produce specific products from chemical reactions using light. If this objective can be achieved, a wide range of technologies would be advanced, from energy conversion (e.g., light to electricity or synthetic fuel) to chemical sensing, to general improvement of chemical process efficiency. In this project supported by the Chemical Structure, Dynamics and Mechanisms-A Program of the Division of Chemistry, Professors Marcos Dantus and Benjamin Levine of Michigan State University are using a combination of experiment and theoretical modeling to design laser light pulses that can result in specific chemical reactions. The light pulses are typically a few femtoseconds in duration (a femtosecond is one-quadrillionth of a second), and can be designed ("shaped") to contain a desired range of light wavelengths (a range of colors), or even change wavelength over the pulse duration. Depending on their shape, the light pulses affect the motions of electrons inside the molecules in different ways. Since electrons form the bonds between the atoms of a molecule, it is possible to control how the bonds break and re-form. In other words, the shape of the laser light pulses can control the outcome of chemical reactions. The graduate and undergraduate students involved in this project learn about light-matter interactions and collaborate with groups that consider these phenomena from different perspectives (spectroscopists theorists, and synthetic chemists). The researchers regularly include high school students in their research efforts and work closely with programs aimed at increasing the number of underrepresented students who pursue graduate study and research careers.

This project implements a novel strategy for achieving coherent control of the energy flow and reactivity of large organic molecules in the condensed phase. Recognizing that different electronic excited states undergo different chemical reactions, shaped laser pulses are being used to (a) populate electronic states with desirable reactivities, and (b) minimize the probability of spontaneous transition out of the desired electronic state (e.g. internal conversion). In pursuit of (b), quantum control strategies that range from semi-classical (driving the vibrational wave packet along a particular reaction coordinate) to quantum strategies with no classical analogue are being used.For example, topological effects near intersections between electronic states can be exploited to influence the reaction outcome and strong coupling, for example when potential energy surfaces are dressed by the light field. In such cases, the natural energy flow is altered and the molecular system?s coherence with the driving field can be enhanced. Advanced quantum dynamical simulations are enabling the determination of causal relationship between the structure of the initial wave packet and reaction outcomes, thus informing subsequent experiments. Successful control of internal conversion are tracked by the fluorescence yield from higher excited states. Subsequently, similar strategies are used to drive dissociative reactions in a series of dyes, which release a highly efficient fluorophore only when excited to a higher excited state. Together, this combined experimental and theoretical effort is elucidating strategies to maximize the fraction of photon energy needed to drive a condensed phase chemical reaction.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) and Chemical Theory, Models, and Computational Methods (CTMC) programs in the Division of Chemistry, Professors Allison, Levine, and Weinacht at Stony Brook University, and Professor Matsika at Temple University are developing new ways to understand the information obtained from sophisticated measurements of the dynamics of molecules. The structure and behavior of molecules are governed by the rules of quantum mechanics. The field of quantum chemistry, which applies the principles of quantum mechanics to molecular problems, has developed over decades based on rigorous comparison between experiments and theory, resulting in reliable computer codes that can be used by non-experts to calculate the properties of molecules in their lowest-energy states. However, similar quantum chemistry calculations are far more challenging for molecules that have been excited, for example by absorbing energy from light, and are able to undergo very fast chemical transformations. Part of the difficulty in developing quantum chemistry methods for excited molecules is that the experimental measurements are much harder to interpret, and comparisons with theory are generally much less rigorous than for molecules in their ground state. This collaborative research team is working to better understand the experimental observables by studying molecules prepared in the same way using different types of experiments, and by making direct comparisons of those observables with quantum chemical calculations that simulate both the measurement process and the excited-state dynamics. In addition to producing a set of benchmark measurements for several representative molecules, the team is working toward a new paradigm for understanding measurements of the dynamics of molecules, including a new format for sharing data. Beyond these scientific broader impacts, the project also provides advanced training for graduate students in a highly collaborative environment.

Ultrafast spectroscopy offers the opportunity to directly probe the dynamics of molecules after excitation. However, the interpretation of data from ultrafast spectroscopy remains a challenge because projection of high dimensional dynamics into a much lower dimensional signal is unavoidable. In principle, a probe that projects the time-dependent molecular wave packet onto the set of all possible states provides a complete, if difficult to interpret, picture of the dynamics in question. The research team led by Professors Allison, Levine, Weinacht, and Matsika is addressing this problem by applying multiple recently developed experimental and theoretical tools to measure and calculate the dynamics of identically prepared gas-phase molecules. Complementary time-resolved photoelectron and visible transient absorption probes project the molecular wave packet onto a broad swath of Hilbert space, providing more information about the dynamics than is possible with either method on its own. The measurements are compared with ab initio simulations of the dynamics, from which identical projections are performed. This rigorous comparison between measured and calculated spectra utilizing complementary probes is enabled by recent methodological advances, including the development of a gas-phase transient absorption spectrometer and novel ab initio tools for efficiently computing probe signals from large molecular dynamics data sets. These systematic studies are producing benchmark datasets on archetypal molecular systems that present challenging problems at the vanguard of quantum chemistry and molecular dynamics, including non-adiabatic dynamics and intersystem crossing. The fundamental processes under investigation play an important role across a wide range of chemical reactions that are driven by light. Through this collaborative effort, the team is also working to develop and disseminate a new data format for sharing both theoretical and experimental ultrafast dynamics results based on the FAIR principle (findable, accessible, interoperable, reusable). Graduate students working on the project learn how to approach complex problems in chemistry based on collaborative research at the forefront of both experiment and theory.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.