Physical Sciences and Engineering

Atomic, Molecular and Optical Physics

 AMO
The atomic, molecular and optical (AMO) physics group explores the frontiers of x-ray science and lays the foundation for x-ray applications in other scientific domains.

The atomic, molecular and optical (AMO) physics group explores the frontiers of x-ray science with a combined theoretical and experimental approach employing advanced synchrotron and x-ray free electron laser sources and Argonne’s large-scale computational facilities.  Advances in wavefront and polarization control of x-ray pulses combined with tailored optical excitation methods offer a new window for understanding and controlling photoinduced molecular interactions in increasingly complex systems.  By aiming for a quantitative and predictive understanding of x-ray interactions with atoms and molecules in gas and condensed phases, both in the linear and nonlinear x-ray interaction regimes, this research lays the foundation for x-ray applications in other scientific domains. 

Nonlinear x-ray interactions

Experimental layout for super-resolution stimulated X-ray Raman spectroscopy where random fluctuations in the intensity and energy of XFELs have been harnessed for high-resolution spectroscopy at the Fourier limit. From Kai Li et al. Nature 643, 662 (2025).

X-ray free electron lasers (XFELs) have spawned an era characterized by multiphoton x-ray interactions with matter. Nonlinear and ultrafast x-ray interaction processes are now accessible, thanks to x-ray pulses with peak brightness a billion times greater than previously available. When nonlinear spectroscopies were developed in the optical regime, a wealth of novel techniques provided new insights on ultrafast photophysical and photochemical processes.  X-ray science is experiencing a similar revolution with the development of stimulated x-ray Raman scattering, x-ray transient grating, three-and four-wave mixing and more. We develop new ways to probe molecules and single particles, with both resonant and non-resonant interactions, by using forefront tools, namely XFELs, upgraded synchrotron sources and high-performance computers. We also design innovative ways to implement these challenging spectroscopic techniques by using the random nature of FEL beams to our advantage with stochastic spectroscopies. 

Ultrafast molecular photophysics

Inner-shell cascade of core-ionized calcium ion in water involving both competing local (Auger) and non-local (Intermolecular Coulombic Decay (ICD) and Electron Transfer Mediated Decay (ETMD)) decays.

Understanding x-ray-initiated processes in isolated and solvated molecules is a grand challenge problem with broad implications for radiation chemistry, physics and biology. High-brightness tunable ultrafast x-ray pulses are now able to track both inner- and outer-shell electronic motion on their natural timescales with chemical site specificity. These time-resolved studies are complemented by precision, coincidence spectroscopies at synchrotrons, like the Advanced Photon Source APS at Argonne. The combination of pump-probe and coincident x-ray experiments allows us to isolate motion prior to inner-shell decay and determine the mechanisms that lead to the final outcomes.  We develop theoretical tools on AURORA, an exascale supercomputer at Argonne Leadership Computing Facility, for high-precision predictions of time-evolving multielectron processes, local and nonlocal inner-decay mechanisms and fragmentation dynamics of heavy-element containing molecules and metal ions in both gas and solution phases.

Chemical dynamics in solution phase

We introduce ultrafast x-ray spectroscopies and electron scattering methods to understand structure and track cavity formation in electron solvation in ionized liquid water. From Arturo Sopena-Moros et al. J. Am. Chem. Soc. 146, 5, 3262-3269 (2024).

We focus on understanding the fundamentals of molecular dynamics in solution induced by optical lasers or x-ray pulses and probed by high-precision, time-resolved x-ray methods. We consider timescales ranging from the attosecond to the microsecond regime, encompassing phenomena ranging from the first steps following photoabsorption triggering electronic dynamics to subsequent processes involving nuclear dynamics. We are especially interested in ligand nuclear dynamics and spin changes in metal complexes. We aim at understanding the solvent and counterion influence on reactivity and on controlling photoinduced outcomes by using feedback loops. Our scientific objectives are pursued using XFELs and the upgraded Advanced Photon Source APS and are complemented by a range of optical techniques available at the Center for Nanoscale Materials CNM. 

Molecular chirality probed by x-rays

This program aims to develop new chirality-sensitive X-ray spectroscopies and use them to probe the local structures and ultrafast dynamics of chiral molecules.

Molecular chirality is an essential stereochemical property of molecules, with relevance in asymmetric catalysis and biochemistry. We aim to use the element-sensitivity of resonant x-rays to probe molecular chirality with local information in molecules. Following an initial theoretical effort [Rouxel J., Mukamel S., Molecular chirality and its monitoring by ultrafast x-ray pulses, Chem. Rev., 2022], experimental demonstrations of these new spectroscopic probes are now being implemented at the APS synchrotron, taking advantage of the new feature beamline POLAR (4ID) that focuses on polarization control. The primary focus is the development of x-ray circular dichroism from liquid-phase molecule, the difference in absorption between left and right circularly polarized x-rays. Alternative techniques involving photoelectrons or nonlinear interactions are also being considered theoretically and experimentally.
This work is supported by the DOE Early Career Research Program.