Excited-state processes from plasma to bioimaging: Theory and applications


Excited-state processes. The two main relaxation channels of an electronically excited molecule are fluorescence and radiationless relaxation, a process in which the system relaxes to the ground state by dissipating electronic energy into heat. Other competing processes, such as transition to a triplet state via inter-system crossing (not shown), excited-state chemistry, and electron transfer, alter the chemical identity of the molecule.

What do plasma, solar panels, qubits, and glow-in-the-dark pigs have in common? The fundamental physics of these phenomena is governed by excited-state processes initiated by light. When a photon is absorbed by a molecule, it promotes an electron to a higher energy level, leading to a new electron distribution that often features an open-shell pattern. This event initiates a variety of processes: radiative and radiationless relaxation, photochemical transformations, electron ejection or attachment. The competition between these processes determines the fate of an electronically excited system—some systems emit light back, some effectively convert excess electronic energy into heat, some produce charge carriers, some change their chemical identity. From the quantum-mechanical point of view, these processes entail coupled electronic and nuclear dynamics. Understanding how these quantum processes unfold in systems with many degrees of freedom, usually coupled to the environment, is of great fundamental importance. Ultimately, we want to know how the chemical structure of the molecule and the environment affect branching ratios and timescales of various excited-state processes. From the practical point of view, the ability to control these processes is the key to the successful design of new photovoltaic materials, bioimaging probes, photodynamic therapies, and materials for high-energy applications (e.g., fusion reactors). Moreover, precise understanding of light-matter interactions and the ability to describe them quantitatively allows us to utilize radiation as a tool for interrogating properties of molecules and materials. Spectroscopy is indeed the most-common and most-powerful tool for deciphering molecular structure. The techniques vary from classic UV-VIS and photoelectron spectroscopies to novel non-linear approaches and high-energy X-ray attosecond pulses.

All these phenomena are governed by the same law: the Schrödinger equation. Tantalizingly simple, it is notoriously difficult to solve. Although quantum chemistry has been very successful in developing practical approaches for the electronic structure problem, challenges still abound. The research of our lab has been driven by fundamental challenges posed by excited-state processes. Specifically, we are pursuing the following directions.

Methods for open-shell and electronically excited species, and approaches for strong correlation. Open-shell character and electronic degeneracies result in multi-configurational wave functions that are not amenable to treatment by the standard single-reference hierarchy of methods. Our group is developing approaches based on a robust and powerful formalism: equation-of-motion coupled-cluster (EOM-CC). We develop new theoretical models for electron correlation, novel algorithms for solving many-body problems, and implement these ideas in practical and efficient computer codes. We are proud to be a part of the Q-Chem open-teamware software project. Quantum chemical methods developed in our group are included in the Q-Chem electronic structure package and are broadly used for calculations of excited states, spectroscopy modeling, magnetic and optical properties of molecular materials and biomolecules, and more. We are also involved in community-wide software development efforts through our partnership with MolSSI.

Relevant publications: EOM-CC methodology, Spin-flip developments

Extending many-body methods to new domains: Resonances and core-level states. Metastable states (such as highly excited autoionizing states, transient anions, and core-ionized states) belong to the continuum part of the spectrum and are thus notoriously difficult to describe theoretically. Our group is developing non-hermitian extensions of EOM-CC theory to treat electronic structure in the continuum.

Relevant publications: Resonances, X-ray

Connection between quantum chemistry and experiment: Spectroscopy modeling. To make the connection between theory and experiment, one needs to be able to go beyond energies and wave functions and model observable properties. We develop tools for computational spectroscopy, ranging from simple models for calculating vibrational progressions to computing total and differential cross-sections in photoelectron/photodetachment experiments by means of Dyson orbitals. Among recent developments are extensions of the theory to model spectroscopy in non-linear and high-energy regimes. We also develop theoretical tools for modeling dynamical processes, such as non-adiabatic relaxation, intersystem crossing, and photo-induced electron transfer.

Relevant publications: Molecular orbital conceps and observables, Spectroscopy and excited-state processes,

Multi-scale methods for extended systems. Multi-scale methods, such as the QM/MM approach, enable rigorous quantum-mechanical treatment of a subsystem (e.g., a chromophore) embedded in an environment (protein, solvent, molecular solid). We are developing multi-scale approaches for modeling condensed-phase processes, such as spectroscopy in solution, photo-induced processes in photo-active proteins, and solar energy harvesting.

Relevant publications: QM/MM, EFP, and beyond Condensed-phase simulations

Applications and collaborations. Parallel to method-development work, we are actively involved in numerous collaborative studies. Our application work can be roughly divided into the following domains:

  1. Bioimaging: Characterizing excited-state properties in fluorescent proteins from the Green Fluorescent Protein family.
  2. Relevant publications: GFP and beyond

  3. Solar energy, photovoltaics, batteries, fuel cells.
  4. Relevant publications: Sustainable energy

  5. Electron transfer in biological systems.
  6. Relevant publications: Charge transfer

  7. Spectroscopy of all shades and flavors for bound and metastable states.
  8. Relevant publications: Dyson, Spectra

  9. Novel materials and quantum information science.
  10. Relevant publications: QIS

  11. Chemistry and spectroscopy of open-shell species.
  12. Relevant publications: Orbital concepts for analysis and quantitative calculations, Spectroscopy modeling, SMMs