X-ray spectroscopy has become a ubiquitous tool in the characterization of new chemical properties and ultrafast molecular dynamics. Core-hole states created by the X-ray interaction have femtoseconds or shorter lifetimes and decay via Auger decay and fluorescence. Auger decay is a form of autoionization involving the collapse of an electron to the core hole which causes the ejection of another to the continuum. The high experimental yields of Auger electrons provide a sensitive probe to selectively monitor the molecular dynamics at specific atomic sites. However, the large number of possible decay pathways leads to a multitude of cationic final states with complex nuclear dynamics. Furthermore, the decay of heavy nuclei leads to Auger decay cascades and thus the interpretation of experimental data is complex. Simulating Auger spectra provides a challenge for quantum chemistry, due to the correlated nature of the decay involving a continuum electron and the large number of final states. Mulliken population analysis methods avoid the computation of two electron integrals with continuum electron wavefunctions and are readily applicable to larger systems and cascade mechanisms. However the accuracy of these approximated methods is limited and quantitively accurate computation of Auger rates are not feasible. A recently developed approach uses a spherically averaged potential of the cation in the numerical solution of the continuum electron wavefunction and can therefore quantitively predict rates of Auger decay. We assess both methods by applying them to multiconfigurational electronic structure calculations and demonstrate their applicability to a variety of cases, including site selective probes, multi-step cascades and population losses in time-dependent dynamics.

Evaluating population analysis methods against the inclusion of the continuum electron wavefunction in auger spectroscopy simulations

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ACS Fall 2022

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