Resonant Auger Decay in Benzene

We present ab initio calculations of the resonant Auger spectrum of benzene. In the resonant process, Auger decay ensues following the excitation of a core-level electron to a virtual orbital. Hence, resonant Auger decay gives rise to higher-energy Auger electrons compared to nonresonant decay. We apply equation-of-motion coupled-cluster (EOM-CC) methods to compute the spectrum in order to explain the main features in the experimental spectrum and to assess the capability and limitations of the available theoretical approaches. The results indicate that participator decay can be well described with the Feshbach-Fano approach based on EOM-CC wave functions in the singles and doubles (SD) approximation, but spectator decay is more difficult to describe. This is because the target states of spectator decay are doubly excited, resulting in the need to include triple excitations in the EOM-CC wave function. Resonant Auger decay in benzene is thus a challenging test case for EOM-CC theory. We examine the performance of different noniterative triple corrections to EOM-IP-CCSD and our numerical results highlight the need to include triple excitations iteratively.