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1. Impacts of hot electron diffusion, electron–phonon coupling, and surface atoms on metal surface dynamics revealed by reflection ultrafast electron diffraction

   https://doi.org/10.1063/5.0205948  

   He, Xing; Ghosh, Mithun; Yang, Ding-Shyue (June 2024, The Journal of Chemical Physics)

   Metals exhibit nonequilibrium electron and lattice subsystems at transient times following femtosecond laser excitation. In the past four decades, various optical spectroscopy and time-resolved diffraction methods have been used to study electron–phonon coupling and the effects of underlying dynamical processes. Here, we take advantage of the surface specificity of reflection ultrafast electron diffraction (UED) to examine the structural dynamics of photoexcited metal surfaces, which are apparently slower in recovery than predicted by thermal diffusion from the profile of absorbed energy. Fast diffusion of hot electrons is found to critically reduce surface excitation and affect the temporal dependence of the increased atomic motions on not only the ultrashort but also sub-nanosecond times. Whereas the two-temperature model with the accepted physical constants of platinum can reproduce the observed surface lattice dynamics, gold is found to exhibit appreciably larger-than-expected dynamic vibrational amplitudes of surface atoms while keeping the commonly used electron–phonon coupling constant. Such surface behavioral difference at transient times can be understood in the context of the different strengths of binding to surface atoms for the two metals. In addition, with the quantitative agreements between diffraction and theoretical results, we provide convincing evidence that surface structural dynamics can be reliably obtained by reflection UED even in the presence of laser-induced transient electric fields. 

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2. Dynamics of nanoscale phase decomposition in laser ablation

   https://doi.org/10.1038/s43246-025-00785-4  

   Sun, Yanwen; Chen, Chaobo; Albert, Thies J; Li, Haoyuan; Arefev, Mikhail I; Chen, Ying; Dunne, Mike; Glownia, James M; Jerman, Martin; Hoffmann, Matthias; et al (December 2025, Communications Materials)

   Abstract Laser ablation is a process that bears both fundamental physics interest and has wide industrial applications. For decades, the lack of probes on the relevant time and length scales has prevented access to the highly nonequilibrium phase decomposition processes triggered by laser excitation. In this study, a close integration of time-resolved probing by intense femtosecond X-ray pulses with large-scale atomistic modeling has yielded unique insights into the ablation dynamics of thin gold films irradiated by femtosecond laser pulses. The emergence and growth of nanoscale density heterogeneities in the expanding ablation plume, predicted in the simulations, are mapped to the rapid evolution of distinct small angle diffraction features. This mapping enables identification of the characteristic signatures of different phase decomposition processes occurring simultaneously in the plume, which are driven by photomechanical and thermodynamic driving forces. Beyond the specific insights into the ablation phenomenon, this study demonstrates the power of joint X-ray probing and atomistic modeling of material dynamics under extreme conditions of thermal and mechanical nonequilibrium. 

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3. Ultrafast x-ray scattering of photodissociation dynamics in 2-iodothiophene

   https://doi.org/10.1063/5.0299432  

   Razmus, Weronika O; Gabalski, Ian; Allum, Felix; Abma, Grite L; Britton, Mathew; Boutet, Sébastien; Bucksbaum, Philip H; Cheng, Xinxin; Crane, Stuart W; Gate, Gregory; et al (January 2026, The Journal of Chemical Physics)

   A time-resolved x-ray scattering (TRXS) investigation of the photodissociation dynamics of gas-phase 2-iodothiophene molecules following 252 nm excitation is presented. Structural evolution of the molecule and dynamical information on the resulting photofragments were captured using femtosecond x-ray free-electron laser pulses. Two dissociation pathways were identified, arising via excitation to ππ* and (n/π)σ* states, respectively, yielding distinct interfragment recoil velocities of ∼6.4 Åps−1 and 17.0 Åps−1. A comparison of asymptotic scattering data with simulated patterns indicates that the thiophene ring remains closed following dissociation at this wavelength. Modeling the experimental data yields a branching ratio of ∼3:1 in favor of the high velocity channel. These findings demonstrate the capability of TRXS to disentangle concurrent ultrafast reaction pathways and provide detailed structural insight into energy redistribution during photoinduced bond fission in complex molecular systems. 

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4. Simultaneous measurements of plasma densities and electron collision times in plasma via time-resolved interferometry

   Nagar, Garima; Dempsey, Dennis; Shim, Bonggu (November 2020, APS Division of Plasma Physics Meeting 2020)

   We present time-resolved interferometry to simultaneously measure plasma densities and electron collision times for strong field laser-matter interactions. First, an intense femtosecond pump pulse generates plasma in a solid and second, a weak 800-nm femtosecond probe traverses the pump-induced plasma and is sent to an interferometer with controlled time delay between pump and probe. By analyzing the interferograms using Fourie methods, we can extract plasma densities and electron collision times in plasma simultaneously with micrometer spatial and femtosecond temporal resolutions. Using the technique, we study the plasma dynamics when a wavelength-varied (λ= 1.2-2.3 μm) pump pulse undergoes laser filamentation in solid materials. 

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5. Panoramic Mapping of Phonon Transport from Ultrafast Electron Diffraction and Scientific Machine Learning

   https://doi.org/10.1002/adma.202206997  

   Chen, Zhantao; Shen, Xiaozhe; Andrejevic, Nina; Liu, Tongtong; Luo, Duan; Nguyen, Thanh; Drucker, Nathan C.; Kozina, Michael E.; Song, Qichen; Hua, Chengyun; et al (January 2023, Advanced Materials)

   Abstract One central challenge in understanding phonon thermal transport is a lack of experimental tools to investigate frequency‐resolved phonon transport. Although recent advances in computation lead to frequency‐resolved information, it is hindered by unknown defects in bulk regions and at interfaces. Here, a framework that can uncover microscopic phonon transport information in heterostructures is presented, integrating state‐of‐the‐art ultrafast electron diffraction (UED) with advanced scientific machine learning (SciML). Taking advantage of the dual temporal and reciprocal‐space resolution in UED, and the ability of SciML to solve inverse problems involving coupled Boltzmann transport equations, the frequency‐dependent interfacial transmittance and frequency‐dependent relaxation times of the heterostructure from the diffraction patterns are reliably recovered. The framework is applied to experimental Au/Si UED data, and a transport pattern beyond the diffuse mismatch model is revealed, which further enables a direct reconstruction of real‐space, real‐time, frequency‐resolved phonon dynamics across the interface. The work provides a new pathway to probe interfacial phonon transport mechanisms with unprecedented details. 

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