Time-Resolved Opportunities at the APS and Beyond: Upgraded Capabilities and Compact X-ray FEL Visions (WK#3 | 2026 APS/CNM user meeting)

Wednesday May 6, 2026, 8:00 AM → 6:00 PM America/Chicago

Building 402 Lecture Hall

Philippe Piot, Xiaoyi Zhang (Argonne National Laboratory)

The APS Upgrade is enabling a new generation of time-resolved X-ray experiments through markedly higher brightness and coherence, alongside active concept studies for a complementary soft X-ray Free-Electron Laser (FEL). This workshop will highlight newly commissioned and emerging pump–probe capabilities at the APS while engaging the user community in defining science-driven requirements for future ultrafast photon sources.

 

 

To attend you need to register to the APS/CNM user meeting here

book of abstracts

8:30 AM → 8:40 AM Opening remarks

Speaker: Jonathan Lang (Argonne National Laboratory)

8:40 AM → 9:05 AM Time resolved PDF investigation of inhomogeneous melting in thin metal films

We performed ultrafast time-resolved diffraction experiments at the x-ray free-electron laser (XFEL) facility in
Pohang, Korea to study 300 nm polycrystalline thin films of gold evaporated onto silicon nitride windows,
melted by a Ti-sapphire laser pulse [1]. A clear splitting of the (111) powder ring was found at certain fluences
after a time delay of 20-100 ps. From the evolution of the x-ray diffraction lineshape, we established separate
roles for the electron and phonon contributions in the melting dynamics, consistent with the prevailing 2-
temperature model. We deduced that the laser energy is primarily taken up by the electrons, which becomes
transmitted into the crystal lattice preferentially at the grain boundaries, converting to heat which diffuses into
the grains and eventually melts them. The appearance of liquid was then tracked by pair-distribution function
analysis and found to have a slight time dependence following melting [2]. We concluded that the melting
process is highly heterogeneous, commencing at the domain boundaries [1]. The role of domain boundaries in
the electrical and mechanical properties of crystals is known from electrical conductivity measurements and
theoretical modelling. This model can be tested on other metals with focussed ultrafast lasers at high power
levels which are sufficient to melt a thin film in a single shot.
[1] Tadesse A. Assefa, Yue Cao, Soham Banerjee, Sungwon Kim, Dongjin Kim, Sunam Kim, Jae Hyuk Lee,
Sang-Youn Park, Intae Eom, Jaeku Park, Daewoog Nam, Sangsoo Kim, Sae Hwan Chun, Hyojung Hyun,
Kyung Sook Kim, Pavol Juhas, Emil S. Bozin, Ming Lu, Changyong Song, Hyunjung Kim, Simon J. L.
Billinge and Ian K. Robinson, Science Advances 6 eaax2445 (2020)
[2] Ian K. Robinson, Jack P. Griffiths, Robert Koch, Tadesse A. Assefa, Ana F. Suzana, Yue Cao, Sungwon Kim,
Dongjin Kim, Heemin Lee, Sunam Kim, Jae Hyuk Lee, Sang-Youn Park, Intae Eom, Jaeku Park, Daewoog
Nam, Sangsoo Kim, Sae Hwan Chun, Hyojung Hyun, Kyung sook Kim, Ming Lu, Changyong Song, Hyunjung
Kim, Simon J. L. Billinge and Emil S. Bozin, IUCrJ 10 656–661 (2023

Speaker: Ian Robinson (Brookhaven National Laboratory)

9:05 AM → 9:30 AM Ultrafast lattice dynamics of metal halide double perovskites by time-resolved X-ray probes

Developing light-emitting semiconductors with high photoluminescence quantum yield (PLQY) is important for
energy-efficient solid-state lighting applications. A subset of solution-processed metal halide perovskites has
been shown to exhibit intrinsic, broadband white-light emission with high PLQY from 10% to 90%. The
intrinsic white-light emission is attributed to self-trapped excitons (STEs), where photo-excited electron-hole
pairs are strongly coupled to local lattice deformations. I will discuss our efforts on understanding light-induced
lattice dynamics underpinning STEs in a prototypical metal halide double perovskite using optical pump-probe
spectroscopy, time-resolved photoluminescence spectroscopy, and optical-pump X-ray diffraction probe
experiments. I will show that charge carrier recombination is asynchronous with lattice dynamics and recovery,
and our hypothesis of a long-lived metastable structure with a recovery time of milliseconds. Additionally, I will
present our recent work on using synchrotron x-ray micro-diffraction to discern the microstructural evolution of
a newly observed spherulite phases from chiral 2D metal halide perovskites.

Speaker: Peijun Guo (Yale)

9:30 AM → 9:55 AM Stochastic and heterogeneous dynamics probed by time-resolved scattering

Photoexcitation by ultrashort laser pulses plays a crucial role in controlling reaction pathways, creating
nonequilibrium material properties, and offering a microscopic view of complex dynamics at the molecular
level. The photo-response following a laser pulse is, in general, non-identical between multiple exposures due to
spatiotemporal fluctuations in a material or the stochastic nature of dynamical pathways. However, most
ultrafast experiments using a stroboscopic pump-probe scheme struggle to distinguish intrinsic sample
fluctuations from extrinsic apparatus noise, often missing seemingly random deviations from the averaged shot-
to-shot response. Leveraging the stability and high photon-flux of time-resolved x-ray micro-diffraction at
Beamline 7-ID-C at the Advanced Photon Source, we employed some established statistical tools to
quantitatively characterize the stochastic behavior of the photoinduced dynamics in a solid-state lithium-based
ionic conductor. By analyzing temporal evolutions of the lattice parameter of a single grain in a powder
ensemble, we found that the sample responses after different shots contain random fluctuations that are,
however, not independent. Instead, there is a correlation between the nonequilibrium lattice trajectories
following adjacent laser shots with a characteristic “correlation length” of approximately 1,500 shots, which
represents an energy barrier of ~0.4 eV for switching the photoinduced pathway, a value that is close to the
activation energy of lithium ion diffusion [1]. I will conclude the talk by discussing new opportunities brought
by this type of analysis to study fluctuations and explore photoinduced metastable states buried in oft-presumed
random, uncorrelated stochastic dynamics.
[1] J. McClellan, A. Zong, et al., “Photoinduced correlations in stochastic dynamics of a solid-state ionic
conductor,” Nature Communications, in press (2026)

Speaker: Alfred Zong (Stanford University)

9:55 AM → 10:15 AM Coffee Break

10:15 AM → 10:40 AM Experimentally detecting subtle drivers in the reactivity of aqueous ferrocyanide

We are using a multi-edge, time-resolved x-ray spectroscopy approach to investigate how the solvent
environment controls reactivity in aqueous iron hexacyanide. This highly charged coordination complex
([Fe(CN)₆]⁴⁻) exhibits useful redox behavior ([Fe²⁺(CN)₆]⁴⁻ ⇌ [Fe³⁺(CN)₆]³⁻) alongside a minor ligand exchange
reaction in which CN⁻ is released and replaced by water. We ask whether systematic tuning of the solvation
shell (a mixture of water molecules and counterions) can control the quantum yield of this photoaquation
channel. Specifically, do particular counterion identities and concentrations bias the fast, collective solvent
response to photodissociated CN⁻ such that geminate recombination is complete, effectively restoring the
starting complex? Conversely, are there conditions under which cage escape is maximized, setting the stage for
downstream bimolecular chemistry such as Prussian blue formation? Key to answering these questions is the
ability to characterize solvation shell structure and dynamics during transient reaction stages. We argue that
multimodal time-resolved X-ray probes are well-poised to provide this information. This talk will present our
efforts using XAS at the Fe K- and L-edges, counterion (K, Na, Ca) K-edges and Fe 1s XES. We anticipate that
probes at the ligand and solvent K-edge and solution scattering (WAXS and time-resolved PDF) will be
powerful additions.

This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Chemical
Sciences, Geosciences, and Biosciences Division.

Speaker: Anne Marie March (Argonne National Laboratory)

10:40 AM → 11:05 AM AI-Driven Spectroscopic Characterization of Defects and Disorder

Defects and structural disorder govern materials functionality, yet their quantitative, non-destructive
characterization remains a major challenge. For defect configuration, we introduce DefectNet, a foundation
model that predicts the chemical identity and concentration of multiple coexisting substitutional point defects
directly from phonon density-of-states spectra. Trained on over 16,000 simulated spectra across 2,000
semiconductors, the model identifies up to six defect species over a wide concentration range and generalizes to
unseen materials. Validation with experimental inelastic scattering data demonstrates accurate, transferable
defect quantification from vibrational spectroscopy.
For defect dynamics, we develop an integrated experiment–theory–AI framework combining coherent x-ray
photon correlation spectroscopy (XPCS) with theory-informed stochastic simulations and semi-supervised
domain adaptation. The approach quantitatively extracts grain-boundary diffusivity, stiffness, and effective
boundary concentration directly from measured two-time correlation functions, overcoming the domain gap
between simulation and experiment. This framework enables robust characterization of slow, non-equilibrium
grain-boundary relaxation in nanocrystalline silicon and provides a general route for bridging theory and
experiment in complex spectroscopic measurements.

Speaker: Mingda Li (Massachusetts Institute of Technology)

11:05 AM → 11:20 AM Upgraded pump-probe capabilities at the APS

This brief presentation will highlight the Time-Resolved Research Group’s new and enhanced capabilities
developed to leverage the APS Upgrade, including pump-probe multimodal imaging and diffraction, grazing-
incidence scattering, pair distribution function measurements, and double-laser-pump TR-XAS. Delivered
immediately before the 40-minute discussion period in the morning session, it is intended to help frame
conversation on how these tools can be most effectively applied to important scientific questions. A central goal
is to inform the community about these capabilities while inviting feedback on priority research directions,
experimental needs, and data challenges that can help guide the future development of time-resolved
capabilities at the APS.

Speaker: Xiaoyi Zhang

11:20 AM → 12:00 PM Discussion

12:00 PM → 1:30 PM Lunch Break

1:30 PM → 1:50 PM Introduction: A Vision for Argonne’s Compact XFEL

Argonne National Laboratory is exploring next-generation x-ray free-electron laser (XFEL) concepts to enable
capabilities beyond existing facilities while complementing the Advanced Photon Source (APS) storage ring. In
this early phase, the focus is on architectures that support multi-user operation and deliver versatile photon-
pulse characteristics, including flexible temporal structure, wavelength tunability, and customizable pulse
formats.
Advanced accelerator concepts under consideration include collinear wakefield acceleration and two-beam
acceleration as potential pathways to compact, high-efficiency XFELs. Synergies with a possible evolution of
the APS injection complex are also being explored to leverage existing infrastructure.
This contribution outlines the emerging vision and key directions and invites community input on photon-pulse
requirements to help guide the development of a next-generation XFEL at Argonne.

Speaker: Philippe Piot

1:50 PM → 2:15 PM Structured Light and X-Ray Pulse Innovation: New Frontiers in XFEL Capabilities

X-ray free-electron lasers (XFELs) are the brightest sources of x-rays available, with a peak brightness that
surpasses table-top harmonic sources and synchrotron radiation facilities by many orders of magnitude.
A defining feature of XFELs is their inherent flexibility, which enables the control of the spectral, spatial, and
temporal properties of the radiation and tailoring of the pulse properties to specific scientific experiments. Since
the early days of LCLS, a vibrant XFEL research and development program has radically changed the way we
do science with XFELs, developing new capabilities such as attosecond pump/probe methods, Terawatt pulses
and seeding and self-seeding techniques.
In my talk I will give an overview of XFEL advances in the last decade and discuss ongoing and future R&D:
from high-brightness cavity-based XFELs to wave-form controlled attosecond pulses.

Speaker: Agostino Marinelli

2:15 PM → 2:40 PM Generating the Fastest X-ray Pulses—Advances in Attosecond Inner-Shell X-ray Laser Science

We present our progress in exploring the phenomenon of stimulated x-ray emission spectroscopy (S-XES)
based on inner-shell lasing at 6 – 8 keV as a new spectroscopy tool, and as a new source of ultrafast hard x-ray
pulses. We first discuss the principle of S-XES, and the experimental methods required for generating and
measuring it. We then discuss recent results for spectroscopy applications of S-XES and its potential and
challenges of providing enhanced electronic structure sensitivity. We then discuss applications of S-XES as a
powerful new x-ray source for probing attosecond dynamics. Here we present the observation of strong lasing
effects and a new technique, x-ray coherent attosecond pulse pair spectroscopy (X-CAPPS) enabling
interferometry for probing phenomena in the 500 attosecond to 5 femtosecond time window. Finally, we
describe the XFEL parameters that would be ideal for advancing the described research.

Speaker: Uwe Bergmann (University of Wisconsin, Madison)

2:40 PM → 3:00 PM Discussion

3:00 PM → 3:20 PM Coffee Break

3:20 PM → 3:45 PM Illuminating Chemical Dynamics: Advancing Reaction Pathways with Future XFEL Capabilities

X-ray free electron lasers (XFELs), with their unique source properties—including femtosecond and sub-
femtosecond pulse durations, high peak brightness, and coherence—offer compelling opportunities for chemical
research. These capabilities allow for the investigation of compounds undergoing photochemical reactions using
site- and element-specific spectroscopy, and, in parallel, for following the corresponding structural changes in
real-time via x-ray scattering. Furthermore, non-linear x-ray spectroscopy methods can provide unparalleled
insights, for instance, into the chemical composition of interfaces. In this presentation, I will provide an
overview of the Linac Coherent Light Source (LCLS), the world’s first hard x-ray XFEL, and detail its ongoing
upgrade. I will also review current directions in chemistry-related research at LCLS and highlight opportunities
afforded by the capabilities of a future next-generation XFEL light source.

Speaker: Thomas Wolf (SLAC National Laboratory & Stanford University)

3:45 PM → 4:10 PM Visualizing dynamics in quantum materials and devices: case for a compact XFEL

In this talk, I will discuss scientific opportunities at the proposed compact XFEL. These opportunities center
around the ultrafast dynamics in quantum materials. These dynamics include changes in the electron
wavefunction and density within atomic clusters or cages, as well as evolutions of charge and magnetic
domains. In both cases, x-ray resonance and polarization will play a central role and yield insights
complementary to that from harder x-ray sources. The interpretation of the experimental data will share
similarities with the chemical sciences, while having challenges of its own. The talk will highlight progress
from XFEL sources around the world in the last decade and discuss unique opportunities going forward.
The work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science,
Basic Energy Sciences, Materials Science and Engineering Division, and through the Early Career Research
Program.

Speaker: Yue Cao (Argonne National Laboratory)

4:10 PM → 4:35 PM Unraveling the Molecular Machinery of Life: Biology with XFELs and Next Generation XFELS

X-rays from XFEL sources have had enormous impact on structural biology both through crystallography and
solution scattering. The new methods demonstrated at XFELs focus on measuring the structural dynamics of
biological macromolecules; these motions enable life. The demonstration of serial femtosecond crystallography,
where tens of thousands of small crystals are individually sampled by single, intense x-ray pulses just prior to
their destruction, enabled measurements on room temperature (as opposed to frozen) samples e.g., [1]. The
ability to gain useful diffraction from micron sized protein crystals also opened up the field of millisecond scale
mix-and-inject serial crystallography [2], where the dynamics of reactions involving proteins, including the
creation of reaction intermediates, could be followed with atomic resolution. Solution scattering also benefits
from the high intensity of XFEL sources. Synchrotron small angle x-ray scattering has proven effective at
determining the low-resolution structures of (randomly oriented) biological molecules in solution, including
measurements on ensembles of structures or time-resolved studies that follow large scale conformational
transitions such as folding. Wide angle x-ray scattering, WAXS has the potential to increase the spatial
resolution of solution studies, but the signal strength can be hundreds to thousands of times weaker than SAXS.
The high intensity of the XFEL sources now enables time-resolved WAXS studies e.g. [3, 4], increasing the
spatial resolution of measurements of structural dynamics. The major theme of these experiments is use of
XFEL x-rays to elucidate the structural dynamics of biomolecules, an essential complement to what is known
about the structures of biomolecules. Next generation XFELs offer more pulses, to offset concerns about sample
consumption as well as novel approaches for experiments on biological molecules.
1. Barends, T.R.M., B. Stauch, V. Cherezov, and I. Schlichting, Serial femtosecond crystallography. Nature
Reviews Methods Primers, 2022. 2(1).
2. Pandey, S., et al., Observation of substrate diffusion and ligand binding in enzyme crystals using high-
repetition-rate mix-and-inject serial crystallography. Iucrj, 2021. 8: p. 878-895.
3. Zielinski, K.A., et al., RNA structures and dynamics with A resolution revealed by x-ray free-electron
lasers. Science Advances, 2023. 9(39).
4. Perera, S.M.D.C., et al., Time-Resolved Wide-Angle X-Ray Scattering Reveals Protein Quake in
Rhodopsin Activation. Biophysical Journal, 2017. 112(3): p. 506a-507a

Speaker: Lois Pollack (Cornell University)

4:35 PM → 5:00 PM Discussion: Closing Discussion