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A theoretical framework for computing Auger spectra that include spin-orbit interaction is presented. The framework is based on the state-interaction approach using equation-of-motion coupled-cluster wave-functions. The working equations for Auger decay rates are derived within the Feshbach–Fano formalism. The capabilities of the theory are illustrated by the calculation of L-edge Auger spectra of H2S and Ar using the Feshbach–Fano and complex basis function (CBF) approaches. The quality of the Feshbach–Fano results depends critically on the treatment of the free-electron state. In contrast to the K-edge spectra for which both plane wave and Coulomb wave treatments yield reasonable results, the Feshbach–Fano calculations yield accurate results for L-edges only when using Coulomb wave (FF-CW). The FF-CW and CBF calculations of Auger spectra in H2S and Ar agree well with each other and with the available experimental data. The results highlight the importance of spin–orbit interactions for modeling L-edge Auger spectra. 

We present an ab initio computational study of the Auger electron spectrum of benzene. Auger electron spectroscopy exploits the Auger–Meitner effect, and although it is established as an analytic technique, the theoretical modeling of molecular Auger spectra from first principles remains challenging. Here, we use coupled-cluster theory and equation-of-motion coupled-cluster theory combined with two approaches to describe the decaying nature of core-ionized states: (i) Feshbach–Fano resonance theory and (ii) the method of complex basis functions. The spectra computed with these two approaches are in excellent agreement with each other and also agree well with experimental Auger spectra of benzene. The Auger spectrum of benzene features two well-resolved peaks at Auger electron energies above 260 eV, which correspond to final states with two electrons removed from the 1 e 1 g and 3 e 2 g highest occupied molecular orbitals. At lower Auger electron energies, the spectrum is less well resolved, and the peaks comprise multiple final states of the benzene dication. In line with theoretical considerations, singlet decay channels contribute more to the total Auger intensity than the corresponding triplet decay channels.