Session K39: DCP Awards

2022 JCP-DCP Future of Chemical Physics Lecture. Talk Title: Dynamic Exciton Polaron in Two-Dimensional Lead Halides

2D lead halide perovskites have shown intriguing properties for optoelectronic applications. Both strongly bound excitons with binding energy up to a few hundreds of meV and polaronic characters have been observed in exciton spectral characters of 2D perovskites but how the interplay between them redefines the nature of exciton polarons and their excited-state behaviors still remains largely unexplored. In this talk, we discuss our recent experimental findings about the exciton polarons in 2D lead halide perovskites and their significant impact on the light emission and charge transfer properties.   

2023 JCP-DCP Future of Chemical Physics Lecture. Talk Title: Towards a big-data ecosystem for quantum chemistry research of solvated molecular systems

Machine learning (ML) and big data play increasingly important roles in both experimental and theoretical studies of chemical physics. Although numerous critical chemical processes occur in the solution phase, datasets (computational or experimental) and machine-learning models for solution-phase molecular systems are still scarce. My research group’s objective is to overcome these challenges by building a big data ecosystem for quantum chemistry research of solvated molecular systems.

 

To enable the efficient generation of computational datasets of solvated molecular systems, we developed strategies to accelerate both the implicit and explicit solvent models for quantum chemistry calculations. For the implicit conductor-like polarization model (C-PCM), we developed algorithms on the graphical processing units (GPUs) to accelerate the calculation. For the explicit solvent model, we developed AutoSolvate, an open-source toolkit to streamline the QC calculation workflow of explicitly solvated molecules.  To improve the accuracy of the generated datasets, we develop ML models to reduce the discrepancy between experimental measurements and computationally predicted molecular properties in both implicit and explicit solvent models. This ML correction technique has been applied to predict redox potential and absorption/fluorescence wavelength in the solution phase.

To make these tools more accessible, we are developing a web-based platform to offer automated simulations of solvated molecules on cloud computing resources and publicly disseminate the datasets to the computational molecular science communities.   

2023 APS Fellow. Talk Title: Picocavity-enhanced ultraresolved molecular spectroscopy and microscopy

A plasmonic nanogap is a superb configuration to explore the interplay between light and matter. Light scattered off, or emitted from a nanogap carries the information of the surrounding electromagnetic environment with it. This situation becomes even more appealing when a single molecule is located in a picocavity, with the molecule playing an active role either in the electromagnetic coupling with the picocavity, or even participating in processes of charge injection and transfer, as revealed through cutting-edge molecular spectroscopy. The process of interaction between a molecular emitter and a nanocavity will be addressed by means of different theoretical frameworks involving aspects of condensed matter physics [1] and quantum chemistry [2]. A battery of methodologies to address ultra resolution in atomic-scale photoluminescence will be described, and many of the theoretical insights obtained will be interpreted in the context of state-of-the-art experimental results in picocavity-enhanced molecular spectroscopy.

[1] A. Babaze, R. Esteban, A. G. Borisov, and J. Aizpurua, Nano Lett. 21, 8466-8473 (2021).

[2] A. Roslawska, T. Neuman, B. Doppagne, A. G. Borisov, M. Romeo, F. Scheurer, J. Aizpurua, and G. Schull, Phys. Rev. X 12, 011012 (2022).   

2023 APS Fellow. Talk Title: Transition State Theory for Nonequilibrium Kinetics: Comparisons with Quasiclassical Trajectories

We consider the applicability and accuracy of bimolecular transition state theory (TST) for cases where the reactants are not in equilibrium with each other. Despite its simplicity, TST has proven itself to be both a means of generating quantitative rate constants as well as the basis of a useful chemical intuition connecting transition state structures with mechanistic insights. In TST, there is an implicit assumption that the transition state is populated according to the same equilibrating assumption as both reactants. When the reactants are not in equilibrium with each other, different assumptions must be made. We derive and test the accuracy of a multi-temperature TST for nonequilibrium kinetics that retains the simplicity of conventional TST. Tests of its accuracy are made via comparisons with quasiclassical trajectories for the kinetics of CH₄ + H, O, and OH, for H₂O + H and O, and for H + HO₂.   

Prize Talk: Earle K. Plyler Prize for Molecular Spectroscopy & Dynamics. Talk Title: X-ray laser probing of ultrafast surface catalysis and the anomalous properties of water

I will demonstrate how x-ray lasers, such as LCLS at the SLAC National Accelerator Laboratory and PAL in Korea, are used to address basic scientific questions in chemical Physics. The first example will illustrate that we can observe surface catalytic reactions in real time using optical pump- x-ray-probe techniques. It has allowed for the detection of transient reaction intermediates in CO desrorption and for probing configurations close to the transition state during CO oxidation both on Ruthenium.

The second example will illustrate how it is now possible to probe water in the deep supercooled regime, where ice crystallization occurs rapidly. Using x-ray laser scattering on ultrafast timescales important features in the behavior of supercooled water could be revealed related to the existence of a liquid-liquid transition around 200 K and 1-2 kbar, and that the thermodynamic response function shows maxima at 230 K and 1 bar. Both observations are only consistent with the existence of a liquid-liquid critical point in deep supercooled water. Such a critical point cause fluctuations extending all the way up to ambient conditions and thereby points to the origin of waters anomalous properties.   

Prize Talk: Justin Jankunas Doctoral Dissertation Award Finalists: Developing a Quantum Chemical Toolbox for Accurate Modeling of K-edges and Beyond

X-ray spectroscopy is a vibrant field that has received considerable attention in recent years due to advances in light sources. By probing the electronic structure of inner-shell orbitals, it enables the investigation of local chemical environments through element specific spectral signatures. From a theoretical perspective, these core-excited states also pose some challenges to existing quantum chemical methods, especially due to lack of relaxation effects that arise after creation of a core-hole. Moreover, as we explore elements of the periodic table with higher atomic numbers, relativistic effects become more relevant, especially for the prediction of K (1s) and L (2p) shell transitions.

In this talk, we discuss different approaches to model X-ray emission (XES) and absorption (XAS), as well as the role of scalar relativistic effects in accurately modeling the core-excitation energies and the spectra of heavy elements. In the context of XES, we analyze the different ingredients that are necessary to accurately model oscillator strengths, and propose a simple metric that provides insights into some of the issues of the traditional linear-response methods, such as time-dependent density functional theory (TDDFT), to model core-spectroscopy.

For XAS, we combine the exact two-component one electron model (X2C) for relativistic effects with two DFT based approaches to study K-edge X-Ray spectroscopies for elements of the third row and early transition metals of the periodic table. The first approach is based on optimizing an excited state configuration directly, within the framework known as orbital-optimized (OO) DFT. The second approach, electron-affinity TDDFT (EA-TDDFT), relies on a double linear-response on top of a core-ionized reference, and attempts to fix the shortcomings of regular TDDFT for core-spectroscopy. Both OODFT and EA-TDDFT, when combined with X2C, show remarkable agreement in predicting K-edge X-Ray absorption (XAS) spectra of a wide range of molecules containing third row main group elements, with RMSE < 0.5 eV. We also investigate how higher order relativistic effects, such as retardation and vacuum fluctuations, play a significant role in the core binding energies of transition metals and how this affects the accuracy of our proposed methods.   

Prize Talk: Justin Jankunas Doctoral Dissertation Award (Finalists). Talk Title: Exciton-Vibration Dynamics Using Real-Time Path Integrals

Over the past two decades, understanding the quantum dynamics of molecular excited states has been a central theme in chemical physics research. Of particular relevance has been the question, "how do nuclear vibrational motions impact the dynamics of inter-molecular excitation energy transfer (EET)?", which has key bearings on our understanding of photosynthetic light harvesting, and on the design of new materials for photovoltaic applications. Numerical simulations of EET are often rendered intractable or severely approximate due to the exponential scaling of quantum mechanics with the combined Hilbert space size of electronic states and nuclear vibrations, and the need for an astronomical number of wave function-based calculations to incorporate thermal effects. During my PhD, I worked on the development of two numerically exact methods based on the Feynman path integral: the modular path integral (MPI) and the small matrix path integral (SMatPI) methods, that allow explicit inclusion of all nuclear normal modes at finite temperatures in EET simulations of large molecular aggregates. Using these methods, we performed the first simulation of the bacterial LH2 complex comprising all 24 excitonic states, and all nuclear normal modes (50 per chromophore) at room temperature, and discovered new aspects about its microscopic quantum dynamics. Particularly, we established that the remarkable (~90%) efficiency and (~1 ps) timescale of EET observed experimentally are enabled by the two-ring arrangement of chromophores, and quantum effects associated with nuclear vibrational motions. We also investigated EET in synthetic aggregates of perylene bisimides and metalloporphyrins, as well as several vibronic models. We illustrated how key features in the ultrafast dynamics of perylene bisimides are modulated by strongly coupled motions of the perylene core. Working with experimentalists, we also showed that small changes in the interplay of electronic and vibrational timescales can speed up exciton relaxation in porphyrin dimers by an order of magnitude, and introduce asymmetric temperature dependent features in the spectra of metalloporphyrin complexes. I will show key glimpses from these stories exhibiting the relevance of nuclear vibrations in quantitatively modulating inter-molecular EET dynamics.