Spectroscopic and Structural Probing of Excited-State Molecular Dynamics with Time-Resolved Photoelectron Spectroscopy and Ultrafast Electron Diffraction

Abstract

Pump-probe measurements aim to capture the motion of electrons and nuclei on their natural timescales (femtoseconds to attoseconds) as chemical and physical transformations take place, effectively making “molecular movies” with short light pulses. However, the quantum dynamics of interest are filtered by the coordinate-dependent matrix elements of the chosen experimental observable. Thus, it is only through a combination of experimental measurements and theoretical calculations that one can gain insight into the internal dynamics. Here, we report on a combination of structural (relativistic ultrafast electron diffraction, or UED) and spectroscopic (time-resolved photoelectron spectroscopy, or TRPES) measurements to follow the coupled electronic and nuclear dynamics involved in the internal conversion and photodissociation of the polyatomic molecule, diiodomethane (CH2I2). While UED directly probes the 3D nuclear dynamics, TRPES only serves as an indirect probe of nuclear dynamics via Franck-Condon factors, but it is sensitive to electronic energies and configurations, via Koopmans’ correlations and photoelectron angular distributions. These two measurements are interpreted with trajectory surface hopping calculations, which are capable of simulating the observables for both measurements from the same dynamics calculations. The measurements highlight the nonlocal dynamics captured by different groups of trajectories in the calculations. For the first time, both UED and TRPES are combined with theory capable of calculating the observables in both cases, yielding a direct view of the structural and nonadiabatic dynamics involved.

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Plain Language Summary

To understand many fundamental processes in physics, chemistry, and biology—such as how organisms cope with molecular damage from sunlight, the basic steps involved in vision, and energy conversion from light—researchers need to be able to follow molecular changes on femtosecond timescales. While much progress has been made on the experimental side, individual techniques typically offer only a limited view of the full dynamics. Here, we combine spectroscopic and structural probes with calculations of the observables to provide a more complete picture of molecular dynamics following the absorption of light.

Specifically, we combine two complementary measurements: ultrafast electron diffraction and time-resolved photoelectron spectroscopy. Each approach to measuring dynamics provides a useful perspective but rarely provides a complete picture on its own. We combine these methods to follow the electronic and nuclear dynamics of the moleculeCH2I2when exposed to UV light.

Our measurements highlight coupled electron-nuclear dynamics that allow for electronic potential energy to be converted into nuclear kinetic energy as well as complicated structural rearrangements of the molecule that involve symmetry breaking, dissociation, rotation, and nonlocal wave-packet dynamics (i.e., the molecule going “two ways at the same time”).

We envision measurements will be more and more combined to yield much greater insight than would be available with either approach alone. Future hardware development could lead to devices that perform both measurements at the same time. We also believe that this work will spur the development of theory and experiment collaborations, particularly in the case where multiple measurements can be performed on the same system.