Structure, spectrum and decomposition of the doubly charged ion C2N2++

The electronic structure and fragmentation of the cyanogen dication formed by double photoionisation of the neutral NCCN molecule, are investigated by means of time of flight photoelectron-photoelectron coincidence (TOF-PEPECO) and photoelectron-photoion-photoion coincidence (PEPIPICO) techniques. Large scale ab initio computations at the cc-pVDZ/RCCSD(T) and cc-pVQZ/CASSCF levels of theory are performed in order to provide detailed structures, harmonic wavenumbers, vertical excitation energies and unimolecular decay pathways of the NCCN++ and CNCN++ isomers. Both theoretical and experimental data show that the NCCN++ dication is metastable; it converts rapidly into the CNCN++ most stable isomer for internal energies above 2 eV. Multi-step dissociation pathways are proposed. The dominant production of ions from unsymmetrical fragmentations is rather unexpected and is explained by rapid rearrangement to the CNCN++ isomer.