Probing Hot Carrier Generation Efficiency in Low-Concentration Plasmonic Gold Nanoparticles via XFEL X-ray Absorption Spectroscopy

Hot carrier generation in plasmonic nanostructures underpins a wide range of energy-conversion technologies, yet its fundamental mechanisms remain elusive due to ultrafast dynamics and detection challenges. This work demonstrates that femtosecond time-resolved X-ray absorption spectroscopy (TR-XAS) at X-ray free-electron lasers (XFELs) enables direct, quantitative probing of hot carrier generation and relaxation in low-concentration gold nanoparticle suspensions. The measured Au L 3 -edge spectra exhibit good agreement with synchrotron references (Pearson correlation coefficient = 0.995), validating the method’s reliability in capturing subtle electronic changes. XFEL-based TR-XAS resolved statistically significant transient signals within 100 fs of localized surface plasmon resonance (LSPR) excitation, allowing for the quantification of hot hole populations down to 0.06 per atom, setting a new sensitivity benchmark for plasmonic systems. By correlating TR-XAS signal intensities with calibrated electronic structure shifts, a precise framework for tracking charge dynamics in metallic nanostructures was established. Furthermore, conducted ab initio calculations, reproduced the obtained experimental data, and provided insight into changes in occupied states expected upon optical illumination in Au NPs. This high-fidelity, element-specific methodology opens new avenues for optimizing hot carrier generation and transfer in plasmon-enhanced applications spanning catalysis, photovoltaics, optoelectronics, and phototherapy.