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Physical Sciences and Engineering

Atomic, Molecular and Optical Physics

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.

X-ray phenomena at the high intensity frontier

Ghost-imaging-enhanced noninvasive spectral characterization of stochastic x-ray free-electron-laser pulses [Kai Li, et. al., Communications Physics 5, 1 (2022). See APS News.]

Understanding ultraintense X-ray interactions with matter is a new frontier with the advent of X-ray free electron lasers (XFELs), which provide femtosecond X-ray pulses of peak brightness a billion-fold greater than those available from synchrotrons.   We examine both single-particle response and propagation through dense media.  We explore this frontier with a concerted experimental and theoretical approach, investigating new nonlinear X-ray phenomena and imaging modalities.  We use forefront tools, namely worldwide X-ray free electron lasers and high performance computers.

Ultrafast X-ray induced phenomena

Ultrashort, intense x-ray pulses can sequentially excite core-electrons from different sites of a molecule, as demonstrated theoretically on the nitrous oxide molecule. [https://​doi​.org/​1​0​.​1​0​8​0​/​0​0​2​6​8​9​7​6​.​2​0​2​2​.​2​1​33749]

Understanding X-ray initiated processes in isolated molecules is a grand challenge problem with broad implications for radiation chemistry, physics and biology. High-brightness tunable ultrafast X-ray pulses, as are available with high-repetition-rate X-ray free electron lasers, will be 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 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.

X-ray probes of condensed phase photoinduced processes

Precision time-resolved x-ray spectroscopy combined with quantum mechanics/molecular mechanics molecular dynamics simulations illuminate details behind a ligand exchange reaction in water.[A.M. March et al., J. Chem. Phys. (2019).]

We focus on understanding the fundamentals of laser-induced phenomena in solution, from the first steps following photoabsorption, with changes in the electronic configuration occurring on the femtosecond time scale, to subsequent processes involved in the evolution of the excited states on the picosecond to microsecond timescales. X-ray spectroscopic probes are used both at synchrotrons and XFELs to track these electronic and structural changes. The upcoming upgraded Advanced Photon Source provides opportunities for high-precision, time-resolved x-ray absorption, emission and Raman spectroscopies at high repetition rate. We have also developed complementary ultrafast optical, infrared transient absorption techniques for these studies. We are especially interested in understanding the influence of the solvent on the reactivity and on influencing the outcome of photoreactions using shaped laser pulses and X-ray feedback.

Recent Publications