Year: 2024
Type: article
Abstract: First principles simulations of excited state X-ray spectra are becoming increasingly important to interpret the wealth of electronic and geometric information contained within femtosecond X-ray absorption spectra recorded at X-ray Free Electron Lasers (X-FELs). However, because the transition dipole matrix elements must be calculated between two excited states (i.e. the valence excited state and the final core-excited state arising from the initial valence excited state) of very different energies, this can be challenging and time-consuming to compute. Herein using two molecules, protonated formaldimine and cyclobutanone, we assess the ability of n-electron valence state perturbation theory (NEVPT2), equation-of-motion coupled cluster theory (EOM-CCSD), linear-response time-dependent density functional theory (LR-TDDFT) and the maximum overlap method (MOM) to describe excited state X-ray spectra. Our study focuses in particular on the behaviour of these methods away from the Franck-Condon geometry and in the vicinity of important topological features of excited-state potential energy surfaces, namely conical intersections. We demonstrate that the primary feature of excited state X-ray spectra is associated with the core electron filling the hole created by the initial valence excitation, a process that all the methods can capture. Higher-energy states are generally weaker and more sensitive to the nature of the reference electronic wavefunction. As molecular structures evolve away from the Franck-Condon geometry, changes in the spectral shape closely follow the underlying valence excitation, highlighting the importance of accurately describing the initial valence excitation to simulate the excited state X-ray absorption spectra.
Authors: Thomas J. Penfold, Basile F. E. Curchod
Institutions: Newcastle University, University of Bristol
Language: English
Subfield: Radiation
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
Awards: UK High-End Computing Consortium for X-ray Spectroscopy (HPC-CONEXS), G4580710395, G4757262767, A Universal Approach for Solving Real-World Problems Using Quantum Dynamics: Coherent States for Molecular Simulations (COSMOS), G660674444, G7962277961, Deep Neural Networks for Real-Time Spectroscopic Analysis
OpenAlex ID: W4405023720