# Double Core Hole Valence-to-Core X-ray Emission Spectroscopy: A Theoretical Exploration Using Time-Dependent Density Functional Theory

## TL;DR

This paper uses time-dependent density functional theory to explore double core hole X-ray emission spectroscopy in transition metal complexes, revealing new insights into local electronic structures beyond traditional methods.

## Contribution

It provides the first theoretical analysis of double core hole valence-to-core X-ray emission spectroscopy using TDDFT, highlighting its potential to reveal detailed electronic information.

## Key findings

- DCH XES signals show energy shifts and intensity changes compared to SCH-XES.
- DCH XES offers additional chemical bonding information around metal centers.
- Predicted DCH-XES features are detectable with current XFEL technology.

## Abstract

With the help of newly developed X-ray free-electron laser (XFEL) sources, creating double core holes simultaneously at the same or different atomic sites in a molecule has now become possible. Double core hole (DCH) X-ray emission is a new form of X-ray nonlinear spectroscopy that can be studied with a XFEL. Here we computationally explore the metal K-edge valence-to-core (VtC) X-ray emission spectroscopy (XES) of metal/metal and metal/ligand double core hole states in a series of transition metal complexes with time-dependent density functional theory. The simulated DCH VtC-XES signals are compared with conventional single core hole (SCH) XES signals. The energy shifts and intensity changes of the DCH emission lines with respect to the corresponding SCH-XES features are fingerprints of the coupling between the second core hole and the occupied orbitals around the DCHs that contain important chemical bonding information of the complex. The core hole localization effect on DCH VtC-XES is also briefly discussed. We theoretically demonstrate that DCH XES provides subtle information on the local electronic structure around metal centers in transition metal complexes beyond conventional linear XES. Our predicted changes from calculations between SCH-XES and DCH-XES features should be detectable with modern XFEL sources.