The N1s shake-up spectrum of para-aminobenzonitrile. A theoretical and experimental ESCA study

A high-resolution ESCA N1s spectrum from free p-aminobenzonitrile molecules is presented. The spectrum is very different from the corresponding spectrum from a solid sample. It shows one uncommonly intense shape-up line from the N1s ionization in the nitrile group, while ionization in the NH₂ group does not give rise to any such anomalous shake-up features. The spectrum is theoretically analyzed by means of a large-scale ab initio calculation by the CAS SCF method, in which a full π-orbital active space was employed for the ground and N1s ionic states. The calculations strongly underline the role of multiple excitations in the charge transfer following N1s(CN) ionization, as well as the importance of separate self-consistent treatment of all states involved. In particular, it is shown that an unsuitable choice of orbitals can lead to an apparent "shake-down".