Phay J. Ho

I study nonlinear and ultrafast x-ray–matter interactions and develop predictive theory and simulations linking ultrafast dynamics to x-ray and experimental observables

Biography

Education

PhD, University of Rochester, New York, USA
BSc, Louisiana State University, Louisiana, USA

Recent Awards

Board of Governors Award on Excellence in Core Values, Sep 2025
Argonne Impact Award for Discovery, Aug 2025
Argonne Impact Award (for notable contribution to the development of Argonne’s Chemical Sciences, Geosciences, and Biosciences Research Strategic Plan for FY 2025-2029), 2024
Argonne Pacesetter Award (for outstanding efforts to increase the effectiveness of postdoctoral mentoring at Argonne), 2019

Research Focus

My research focuses on the fundamental interaction of intense, ultrafast x-ray radiation with matter, with an emphasis on nonequilibrium electronic and nuclear dynamics in atoms, molecules, and complex systems. I develop first-principles theoretical frameworks, large-scale computational models, and data-driven approaches to understand how intense x-ray fields drive excitation, ionization, relaxation, and structural evolution.

A central goal of my work is to establish quantitative links between microscopic dynamics and experimentally measurable signals, including scattering, spectroscopy, and charged-particle observables. This research integrates atomic and molecular physics, electronic structure theory, quantum optics, high-performance computing, and machine learning, enabling predictive modeling of ultrafast x-ray experiments at synchrotron and x-ray free-electron laser (XFEL) facilities.

Ultrafast and Nonlinear X-ray–Matter Interactions

We investigate how strong x-ray fields drive electronic dynamics beyond the linear-response regime, where resonant, nonlinear, and coherence effects compete with inner-shell decay processes. Our theory work focuses on regimes in which transient core excitation, stimulated processes, and strong-field dynamics fundamentally reshape x-ray absorption, emission, scattering, and propagation, modifying both signal formation and decay pathways.

We develop theoretical frameworks grounded in quantum optics to explore atomic-scaling imaging and nonlinear x-ray spectroscopy strategies that mitigate radiation damage while enhancing signal strength through coherent quantum pathways, interference effects, and x-ray–driven Rabi dynamics in transient core-excited states. These frameworks aim to enable the systematic exploration of quantum pathways and interference effects, and to provide predictive understanding of imaging and spectroscopic signatures in the intense x-ray field regime. Together, these approaches enable access to atomic-scale, transient, and nonequilibrium structure on femtosecond and attosecond timescales, including resonance-enhanced diffractive imaging and fluorescence intensity–correlation (Hanbury Brown–Twiss–based) imaging, with signal amplification beyond linear-response limits.

Ultrafast X-ray Molecular Photophysics and Solvation Chemistry

A major theme of our research is the time-dependent evolution of inner-shell–excited states. We study core-hole formation, orbital relaxation, charge redistribution, and multistep local and nonlocal decay cascades, and their coupling to nuclear motion in isolated and solvated molecules and ions. Understanding x-ray–induced processes in molecular environments is broadly relevant, with applications spanning radiolysis, structural biology, medical therapies, and astrophysics.

By explicitly treating many-electron dynamics and ultrafast relaxation, our work reveals how energy flow, charge redistribution, and chemical pathways are initiated following x-ray absorption in heavy-element and chemically complex systems. A current focus is the time evolution of solution-phase dynamics across multiple length and time scales, including local and nonlocal electron excitation, transfer, and decay mechanisms, and how these processes drive environment-dependent fragmentation, solvated-electron formation, and ultrafast solvent rearrangement in locally excited molecules and metal ions.

Molecular Structure and Dynamics from Coulomb Explosion Imaging

Coulomb explosion imaging (CEI), first developed using ultrafast electron stripping of fast molecular ions and later extended to strong-field laser and ultrafast x-ray excitation, maps molecular structure and dynamics onto correlated electron and ion emission following intense ionization.

We develop theoretical and computational frameworks that connect electronic ionization and relaxation pathways to the ensuing nuclear breakup, providing a quantitative basis for interpreting CEI experiments and enabling structure retrieval from electron–ion coincidence measurements under extreme x-ray conditions. These first-principles models describe CEI signal formation and generate high-fidelity training data, enabling the collaborative development of MOLEXA, a generative machine-learning framework for molecular structure reconstruction. Within this hybrid physics–ML approach, data-driven models accelerate large-scale simulations and enable rapid inversion of high-dimensional observables in CEI and nonlinear x-ray scattering.

Theory, Simulation, and High-Performance Computing

Our research relies on scalable computational frameworks that couple electronic dynamics, ionization kinetics, and nuclear motion to model ultrafast x-ray–matter interactions from first principles. These methods are designed for massively parallel high-performance computing architectures, and we have led multiple leadership-class simulation campaigns supported by DOE allocations. This combination of physics-based theory, advanced algorithms, and large-scale computing enables predictive modeling at system sizes and levels of realism that are inaccessible to conventional approaches.

A central focus is the development of accurate electronic-structure and many-electron dynamical methods that incorporate core excitation, relativistic effects, and fully quantum descriptions of multistep relaxation and molecular dynamics. These capabilities are essential for predictive modeling of heavy-element systems, complex molecular environments, and strong-field x-ray regimes.

Representative methods developed and employed in our work include:

Quantum Decay-Spawning Dynamics for multistep inner-shell decay, electron transfer, and coupled nuclear dynamics in molecules
Time-dependent QED-based scattering theory for quantum pathways and interference in resonant x-ray scattering of few-atom systems
Density-matrix and open-quantum-system approaches for x-ray–driven coherence, relaxation, and decoherence
Monte Carlo / Molecular Dynamics (MC/MD) simulations of x-ray–driven dynamics and scattering in clusters
MC/MD combined with classical over-the-barrier (COB) models for molecular x-ray excitation and charge redistribution
Monte Carlo rate-equation approaches for intense x-ray atomic ionization
Time-dependent configuration-interaction singles (TDCIS) and related wavefunction-based methods for strong-field ionization and high-harmonic generation dynamics in the optical regime
QED-based theory of x-ray diffraction from laser-aligned molecules, including rotational wave-packet dynamics and finite x-ray coherence

Earlier in my work, including Ph.D.-level research, I developed quantum, hybrid, and classical approaches to study strong-field ionization and correlated multielectron dynamics, including nonsequential double, triple, and quadruple ionization in intense optical fields.

Recent Publications

Nivedita Bhat, Yeonsig Nam, Linda Young, Stephen H. Southworth, and Phay J. Ho, Impact of Atomic Substitution on Core-Hole Relaxation Dynamics: A Study of Br2 and IBr, accepted to J. Chemical Physics as Communications (2026)
Xubo Wang, Phay J. Ho, Chaoqun Zhang, and Lan Cheng, Relativistic Exact Two-Component Theory in the Generalized Pseudospectral Representation, submitted to PRA (2026).
Phay J. Ho, Jeremy Rouxel and Linda Young, Attosecond imaging, radiolysis and chiral dynamics, submitted to J. of Optics,(2026)
Xiang Li, Till Jahnke, Rebecca Boll, Jiaqi Han, Minkai Xu, Michael Meyer, Maria-Novella Piancastelli, Daniel Rolles, Artem Rudenko, Florian Trinter, Thomas J.A. Wolf, Jana B. Thayer, James P. Cryan, Stefano Ermon and Phay J. Ho, Generative Modeling Enables Molecular Structure Retrieval from Coulomb Explosion Imaging, submitted to Nat. Comm. (2026).
A. Venkatesh and P. J. Ho , Quantum Interference in Two-Atom Resonant X-ray Scattering of an Intense Attosecond Pulse, submitted to PRA, arXiv:2506.06585 (2026).
A. Ulmer, P. J. Ho, B. Langbehn, S. Kuschel, L. Hecht, R. Obaid, S. Dold, T. Driver, J, Duris, M.-F. Lin, D. Cesar, P. Franz, Z. Guo, P. A. Hart, A. Kamalov, K. A. Larsen, X. Li, M. Meyer, K. Nakahara, R. G. Radloff, R. Robles, L. Rönnebeck, N. Sudar, A. M. Summers, L. Young, P. Walter, J. Cryan, C. Bostedt, D. Rupp, A. Marinelli, and T. Gorkhover, Nonlinear reversal of photo-excitation on the attosecond time scale improves ultrafast x-ray diffraction images, submitted(2026), https://arxiv.org/abs/2506.19394
P. J. Ho, T. Gorkhover and L. Young, Prospects of Studying Inner-shell Orbital Relaxation with Attosecond X-ray Pulses, submitted (2026).
Adam EA Fouda, Bhavnesh Jangid, Eetu Pelimanni, Stephen H Southworth, Phay J Ho, Laura Gagliardi, Linda Young, “Computation of Auger Electron Spectra in Organic Molecules with Multiconfiguration Pair-Density Functional Theory”, J. Phys. Chem A 129, 8419-8431 (2026). https://doi.org/10.1021/acs.jpca.5c01789 (Aug)
Kai Li, Christian Ott, Marcus Agåker, Phay J Ho, Gilles Doumy, Alexander Magunia, Marc Rebholz, Marc Simon, Tommaso Mazza, Alberto De Fanis, Thomas M Baumann, Jacobo Montano, Nils Rennhack, Sergey Usenko, Yevheniy Ovcharenko, Kalyani Chordiya, Lan Cheng, Jan-Erik Rubensson, Michael Meyer, Thomas Pfeifer, Mette B Gaarde, Linda Young, “Super-resolution stimulated X-ray Raman spectroscopy”, Nature 643, 662-668 (2025). https://doi.org/10.1038/s41586-025-09214-5 (July)
Xiang Li, Rebecca Boll, Patricia Vindel-Zandbergen, Jesús González-Vázquez, Daniel E Rivas, Surjendu Bhattacharyya, Kurtis Borne, Keyu Chen, Alberto De Fanis, Benjamin Erk, Ruaridh Forbes, Alice E Green, Markus Ilchen, Balram Kaderiya, Edwin Kukk, Huynh Van Sa Lam, Tommaso Mazza, Terence Mullins, Björn Senfftleben, Florian Trinter, Sergey Usenko, Anbu Selvam Venkatachalam, Enliang Wang, James P Cryan, Michael Meyer, Till Jahnke, Phay J Ho, Daniel Rolles, Artem Rudenko, “Imaging a light-induced molecular elimination reaction with an X-ray free-electron laser”, Nature Communications 16, 7006 (2025). https://doi.org/10.1038/s41467-025-62274-z (Jul)
Akilesh Venkatesh, Phay J Ho, “Theory of resonant x-ray scattering with ultrafast intense pulses”, Physical Review A 111, 023101 (2025). https://doi.org/10.1103/PhysRevA.111.023101
Akilesh Venkatesh, Phay J Ho, “Effect of Rabi dynamics in resonant x-ray scattering of intense attosecond pulses”, Phys. Rev. A 111, L021101 (2025). https://doi.org/10.1103/PhysRevA.111.L021101 (Feb)
Stephan Kuschel, Phay J Ho, Andre Al Haddad, Felix F Zimmermann, Leonie Flueckiger, Matthew R Ware, Joseph Duris, James P MacArthur, Alberto Lutman, Ming-Fu Lin, Xiang Li, Kazutaka Nakahara, Jeff W Aldrich, Peter Walter, Linda Young, Christoph Bostedt, Agostino Marinelli, Tais Gorkhover, “Non-linear enhancement of ultrafast X-ray diffraction through transient resonances”, Nature Communi. 18, 847 (2025). https://doi.org/10.1038/s41467-025-56046-y (Jan)
E. Pelimanni, A. E. A. Fouda, P. J. Ho, T. M. Baumann, S. I. Bokarev, A. De Fanis, S.Dold, G. Grell, I. Ismail, D. Koulentianos, T. Mazza, M. Meyer M. N. Piancastelli, R. Püttner, D. E. Rivas, B. Senfftleben, M. Simon, L. Young and G. Doumy “Observation of molecular resonant double-core excitation driven by intense X-ray pulses”, Comm. Phys. 7 341 https://doi.org/10.1038/s42005-024-01804-5
Adam E. A. Fouda, Stephen H. Southworth, Phay J. Ho “Quantum Molecular Charge-Transfer Model for Multi-step Auger-Meitner Decay Cascade Dynamics”, J . Chem. Theory Comput. 20, 8782 (2024). https://doi.org/10.1063/5.0145215
A. E. A. Fouda, V. Lindblom, S. H. Southworth, G. Doumy, P. J. Ho, L. Young, L. Cheng and S. L. Sorensen “Influence of Selective Carbon 1s Excitation on Auger–Meitner Decay in the ESCA Molecule”, J. Phys. Chem. Lett. 15, 4286-4293 (2024).
R.B. Weakly, C.E. Liekhus-Schmaltz, B.I. Poulter, E. Biasin, R. Alonso-Mori, A. Aquila, S. Boutet, F.D. Fuller, P. J. Ho, T. Kroll, C.M. Loe, A. Lutman, D. Zhu, U. Bergmann, R. W. Schoenlein, N. Govind, M. Khalil, Revealing core-valence interactions in solution with femtosecond X-ray pump X-ray probe spectroscopy, Nat. Commun. 14, 3384 (2023).
Phay J. Ho, “Bringing Interferometric Imaging into the X-Ray Regime.” Physics 16, 66 (2023). https://physics.aps.org/articles/v16/66
Phay J. Ho, Dipanwita Ray, C. Stefan Lehmann, Adam E. A. Fouda, Robert W. Dunford, Elliot P. Kanter, Gilles Doumy, Linda Young, Donald A. Walko, Xuechen Zheng, Lan Cheng, Stephen H. Southworth, “X-ray induced electron and ion fragmentation dynamics in IBr”. J. Chem. Phys. 158, 134304 (2023). https://doi.org/10.1063/5.0145215
A. Al-Haddad, S. Oberli, J. Gonzalez-Vazquez, M. Bucher, G. Doumy, P. J. Ho, J. Krzywinski, T. J. Lane, A. Lutman, A. Marinelli, T. J. Maxwell, S. Moeller, S. T. Pratt, D. Ray, R. Shepard, S. H. Southworth, A. Vazquez-Mayagoitia, P. Walter, L. Young, A. Picon, and C. Bostedt, “Observation of site-selective chemical bond changes via ultrafast chemical shifts”, Nat. Comm. 13, 7170 (2022). https://doi.org/10.1038/s41467-022-34670-2
L. Gavilan, Phay J. Ho, U. Gorti, H. Ogasawara, C. Jager, F. Salama, “A laboratory-driven multiscale investigation of X-ray induced mass loss and photochemical evolution in cosmic carbon and silicate dust,”Astrophys. J. 925, 86 (2022). https://doi.org/10.3847/1538-4357/ac3dfd
Adam E A Fouda, Dimitris Koulentianos, Linda Young, Gilles Doumy and Phay J. Ho, “Resonant Double-Core Excitations with Ultrafast, Intense Pulses,” Mol. Phys., 121, e2133749 (2022). https://doi.org/10.1080/00268976.2022.2133749
Phay J Ho, Christopher Knight, Linda Young, “Fluorescence intensity correlation imaging with high spatial resolution and elemental contrast using intense x-ray pulses,” Structural Dynamics 8, 044101 (2021).https://doi.org/10.1063/4.0000105
Chelsea E Liekhus-Schmaltz, Phay J Ho, Robert B Weakly, Andrew Aquila, Robert W Schoenlein, Munira Khalil, Niranjan Govind, “Ultrafast x-ray pump x-ray probe transient absorption spectroscopy: A computational study and proposed experiment probing core-valence electronic correlations in solvated complexes,” J. Chem. Phys. 154, 214107 (2021). https://doi.org/10.1063/5.0047381
Adam E. A. Fouda, Phay J. Ho, “Site-specific generation of excited state wavepackets with high-intensity attosecond x rays,” J. Chem. Phys. 154 (22), 224111 (2021). https://doi.org/10.1063/5.0050891