Improved Charge Transfer Multiplet Method to Simulate M- and L-Edge X-ray Absorption Spectra of Metal-Centered Excited States

TL;DR

This paper extends the charge transfer multiplet (CTM) method to simulate M- and L-edge X-ray absorption spectra of excited states in transition metal complexes, enabling better interpretation of experimental spectra and electronic states.

Contribution

The authors develop new features in the CTM4XAS software to simulate excited state spectra and interpret eigenfunctions, advancing the application of CTM theory to excited state analysis.

Findings

Reinterpreted ferrocenium cation spectra with new assignments.

Modeled FeII spectra during spin crossover, identifying vibrational signatures.

Extended CTM4XAS to excited states, improving spectral analysis.

Abstract

Charge transfer multiplet (CTM) theory is a computationally undemanding and highly mature method for simulating the soft X-ray spectra of first-row transition metal complexes. However, CTM theory has seldom been applied to the simulation of excited state spectra. In this article, we extend the CTM4XAS software package to simulate M2,3- and L2,3-edge spectra of excited states of first-row transition metals and to interpret CTM eigenfunctions in terms of Russell-Saunders term symbols. We use these new programs to reinterpret the recently reported excited state M2,3-edge difference spectra of photogenerated ferrocenium cations and propose alternative assignments for the electronic state of the photogenerated ferrocenium cations supported by CTM theory simulations. We also use these new programs to model the L2,3-edge spectra of FeII compounds during nuclear relaxation following photoinduced spin crossover, and propose spectroscopic signatures for their vibrationally hot states