Collaborative Research: Understanding Ultrafast Observables

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.