Potential Energy Surfaces of Core-Hole and Shake-Up States for Dissociative Ionization Studies

The theoretical description of core-hole and core-hole excited (shake-up) states resulting from the interaction of a molecule with X-ray free-electron lasers, attosecond pulses, and synchrotron radiation is a challenging task, as these states lie in the ionization continuum and, therefore, are subject to variational collapse. Although much effort has been devoted in the past to describe core-hole states in the Franck-Condon (FC) region, very few attempts, even less for shake-up states, have been reported near the dissociation threshold, where multistate degeneracy introduces additional complications. Knowledge of the whole potential energy surface (PES) is, however, essential when dissociative ionization resulting from the interaction with XUV or X-ray light occurs. In this work, we gauge the accuracy of two widely used multiconfigurational methods, CASSCF and MRCI, to obtain the potential energy curves of core-hole states of CO, N₂, and the small polyatomic molecule BF₃, from the FC region up to the dissociation limit, and show that CASSCF in combination with a triple-ζ basis set provides a very reasonable compromise between accuracy and computational cost to correctly describe core-hole and shake-up states in the entire PES. Besides providing the vibrational spectroscopic constants associated with these states, we also analyze the corresponding bonding and dissociation properties, which, in some cases, are counterintuitive and significantly different from those of bound states.