Progress in the Theory of X-ray Spectroscopy: From Quantum Chemistry to Machine Learning and Ultrafast Dynamics

Abstract

The development of high-brilliance third- and fourth-generation light sources such as synchrotrons and X-ray free-electron lasers (XFELs), the emergence of laboratory-based X-ray spectrometers, and instrumental and methodological advances in X-ray absorption (XAS) and (non)­resonant emission (XES and RXES/RIXS) spectroscopies have had far-reaching effects across the natural sciences. However, new kinds of experiments, and their ever-higher resolution and data acquisition rates, have brought acutely into focus the challenge of accurately, quickly, and cost-effectively analyzing the data; a far-from-trivial task that demands detailed theoretical calculations that are capable of capturing satisfactorily the underlying physics. The past decade has seen significant advances in the theory of core-hole spectroscopies for this purpose, driven by all of the developments above andcruciallya surge in demand. In this Perspective, we discuss the challenges of calculating core-excited states and spectra, and state-of-the-art developments in electronic structure theory, dynamics, and data-driven/machine-led approaches toward their better description.

Enhancing the analysis of disorder in X-ray absorption spectra: application of deep neural networks to T-jump-X-ray probe experiments

“…4.5 Â 10 À2 (evaluated against unseen/'out-ofsample' spectra for which the post-edge has been normalised to unity), demonstrating two-fold-improved performance relative to our previous work at the Fe K-edge. 8,46,47 This is due -in partto the larger dataset that we work with here (40 700 local geometry/ spectrum pairs at the Co K-edge vs. 9040 local geometry/spectrum pairs at the Fe K-edge). Fig.…”

“…[4][5][6] X-ray spectroscopy (XS) has similarly broad applications as it is able to deliver detailed information about electronic and geometric structure and, in particular, the local environment around the X-ray absorption site. 7,8 In T-jump pump/X-ray probe experiments, T-jump spectroscopy and XS are coupled together to deliver direct structural insight into thermally-activated chemical reactions; such experiments have, for example, been used to study the response of the structure of water following the transfer of heat via NIR excitation. 9,10 The use of X-ray absorption near-edge structure (XANES) [11][12][13][14][15][16][17] and/or extended X-ray absorption fine structure (EXAFS) [18][19][20][21][22][23] measurements to characterise solvent and/or solvation structures is well established.…”

“…To address this, contemporary works have explored supervised machine learning/deep learning algorithms with a view towards mapping the relationship between XANES spectra and the electronic and geometric structures of the systems that they characterise. 8,[33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] For an ab initio MD-based approach like that described in the present Article, our own deep neural network (DNN; introduced in ref. 46) could be used to accelerate the prediction of the X-ray spectra for each of the ab initio MD snapshots (the bottleneck of the strategy), opening up a fast and cost-effective route to the quantitative interpretation of T-jump pump/X-ray probe experiments.…”

Perspective on integrating machine learning into computational chemistry and materials science

“…For instance, in case of spectroscopic predictions it is sufficient to learn the spectral shape directly instead of the energy levels. This has been done with Gaussian Approximation Potentials for the densityof-states 83 and with NNs for X-ray spectroscopy 85,86 or for excitation spectra. 74 In the latter case, NNs could describe spectral intensities with deviations of 0.03 arb.u..…”

Broadband X-Ray Absorption Spectra from Time-Dependent Kohn-Sham Calculations

“…Quantum chemistry is currently witnessing a resurgence of interest in x-ray spectroscopy, [1][2][3][4][5][6][7] catalyzed by the emergence of new technologies including coherent ultrahigh harmonic generation, 8 providing capabilities for ultrafast time resolution at x-ray wavelengths, [9][10][11] even with tabletop laser systems. 12 This technology has enabled x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) studies of solution-phase systems, [13][14][15][16] as well as surface-sensitive ultrafast spectroscopy at extreme ultraviolet (XUV) wavelengths.…”