Research

The Keefer group implements and uses simulation protocols for the quantum dynamics, ultrafast spectroscopy, and quantum control of molecules. The general paradigm is to theoretically design and predict novel means to measure and manipulate molecular motion with light. We accomplish this by first obtaining fundamental mechanistic insight into light-induced chemistry through quantum dynamical simulations. We then develop and compute spectroscopic signatures that are sensitive to the key molecular features and attempt to steer molecular motion into otherwise inaccessible quantum pathways. Beyond our central research focus, we foster curiosity-driven explorations into other areas where our ideas and methods can make meaningful contributions, such as quantum information.

We are fortunate to be funded by the European Research Council (ERC) and the Max Planck Society.
 

Quantum Molecular Dynamics

We employ a simulation framework that solves the time-dependent Schrödinger equation exactly within a reduced-dimensional coordinate space. This approach relies on finding a few “reactive” nuclear degrees of freedom that dominate the molecular dynamics in the electronic ground and excited states. The key advantage is that we have access to the full nuclear and electronic wave function in this low-dimensional space, allowing us to identify the key quantum mechanical phenomena of a photo-induced molecular process. This is particularly useful when simulating the passage of molecular wave packets through conical intersections, a process we also call “photochemical decision making”. Complementary to exact quantum dynamics, we also employ mixed quantum-classical and fully classical molecular dynamics simulations, extending our simulation toolbox from the attosecond to the nanosecond timescale, with a focus on femtosecond excited-state molecular dynamics.

Ultrafast Spectroscopy

We simulate the spectroscopic interrogation of molecular dynamics with ultrashort laser pulses. In analogy to a video camera, by varying the time delay(s) between individual pulses, stroboscopic frames of information are concatenated to form a “molecular movie”. Our goal is to theoretically conceptualize movies that have not yet been recorded. A particular focus is on X-ray sources from free-electron lasers and high harmonic generation setups, which open new windows into molecular dynamics with unprecedented temporal, spectral and spatial resolution. We actively collaborate with experimental groups to find practical ways to realize our ideas and to provide interpretations for cutting-edge measurements.

Quantum Optimal Control

Whenever a spectroscopic signal is recorded, the full-dimensional molecular wave function is projected onto a lower-dimensional observable. Especially for coupled nuclear and electronic motions, important dynamical signatures remain elusive due to their intrinsic weakness or because they spectrally overlap with much stronger but less interesting contributions. We use quantum optimal control to selectively enhance and isolate these signatures. The amplitudes of the individual spectral components of the pump and probe laser pulses are modulated to modify and optimize the spectroscopic projections of wave functions onto observables.

In addition to spectroscopy, quantum optimal control is a key technique in quantum computing for implementing quantum logic operations in qubit systems.