Computation of Partial Auger Decay Widths from Complex-Valued Equation-of-Motion Coupled-Cluster Energies

We discuss the computation of partial Auger decay widths with equation-of-motion ionization-potential coupled-cluster (EOMIP-CCSD) theory in the framework of non-Hermitian quantum mechanics (NHQM). In NHQM, the decaying character of metastable states is described with complex energies and the total decay width is obtained directly from the total energy. In contrast, the computation of partial decay widths, i.e., the contributions of different decay channels to the total width, requires further analysis. However, partial widths are important for Auger spectroscopy as they determine the probability with which different final states are formed and hence the shape of the Auger spectrum. Recently, we introduced Auger channel projectors (ACPs), which selectively remove decay channels from the EOMIP-CCSD excitation manifold. This method requires a separate EOMIP-CCSD calculation for each decay channel. Here, we suggest an alternative: We solve the EOMIP-CCSD equations for the core-ionized state in the full excitation manifold and decompose the imaginary part of the resulting energy. In this way, we obtain all partial decay widths at once. We compute Auger spectra for K-edge-ionized states of methane, ethane, and hydrogen sulfide, and a Coster-Kronig spectrum for L₁-edge-ionized hydrogen sulfide. The results obtained with our new approach differ only negligibly from ACP results. We also present the first Auger spectra for the cyanide anion, including vibrational broadening, and discuss the differences between the spectra of the carbon core hole and the nitrogen core hole.