Description
Recent advances at XFEL facilities have enabled the use of X-ray phtoelectron spectroscopy (XPS) as a probe in time-resolved measurements. Transient XPS spectroscopy combines high temporal resolution with exceptional sensitivity to local chemical environment, paving the way to study ultrafast, localized chemical changes with the element-specific XPS technique. Several joint experimental and theoretical studies on transient XPS of small molecular systems have recently been reported with remarkable temporal and energy resolution, e.g. ref. (1, 2 ), demonstrating the usefulness of transient XPS for characterizing systems and processes of chemical interest.
A transient XPS spectrum is typically modeled by combining ab-initio molecular dynamics (to evolve the molecular structure of the chemical system at hand) with an explicit calculation of the XPS spectrum associated to each molecular geometry involved in the molecular dynamics (3).
In this contribution, a systematic assessment of partial charge models for simulating transient XPS across several molecular systems will be presented and discussed. Key open questions remained to be addressed, such as: How can atomic partial charges be computed effectively and reliably? Could this model be useful not only to locate the main transient XPS signal, but also the satellites? And could the model be extended beyond core-ionization at the K edges of first-row elements to other regions of the periodic table?
Further investigation into the reliability of partial charge models could provide scientists with an approach that is not only computationally efficient but also rich in physical insight. Such models can reveal the influence of local charges, the role of vibrational dynamics, the correlation between electronic states and nuclear motion, and the importance of electronegativity. Currently, the explicit calculations of electronic wavefunctions for numerous core-ionized states represents the main computational bottleneck in simulating transient XPS spectra. A robust partial charge model would overcome this limitation, enabling simulations of transient XPS spectra of larger and more complex chemical systems. This advancement would significantly broaden the applicability of transient XPS, extending its reach to processes of greater chemical relevance and enhancing the impact of this cutting-edge technique.
References
1. D. Faccial`a et al., Journal of the American Chemical Society 147, 30694– 30707, (https://doi.org/10.1021/jacs.5c04874) (2025).
2. H. J. Thompson et al., Journal of the American Chemical Society 147, 32851–32860, (https://doi.org/10.1021/jacs.5c09162) (2025).
3. D. E. Rivas et al., Physical Review X 16, 011051, (https://link.aps.org/doi/10.1103/y6dt-1sfw) (1 2026).
