Since benzonitrile’s discovery in the interstellar medium (ISM) in 2018, several studies have explored the strongest unimolecular dissociations of its radical cation (C6H5CN•+). However, sequential dissociation processes, which become important when ionization occurs with significant excess energy transfer, have received almost no attention to date. The present metastable dissociative ionization experiments reveal 14 different dissociations, of which 11 have never been observed before. Nine of these new reactions involve the dissociation of a fragment ion. A notable result shows that C4H2•+ production (the second most intense fragment ion in conventional mass spectra without metastable dissociation analysis) derives from sequential dissociation via C6H4•+, as well as from the previously reported unimolecular dissociation of C6H5CN•+. Furthermore, our experiments demonstrate new pathways that produce astrochemically important neutrals including HCN/CNH and CN•, as well as revealing CH• and C3H• production from ionized benzonitrile for the first time. In addition to the metastable dissociation experiments, we applied density functional theory to calculate two sequential dissociation routes and report the results of our detailed analysis of the peak shapes in a conventional mass spectrum of benzonitrile. The latter enabled the dominant ion to be identified in peaks with nearest-integer m/z values that match two conceivable ions. The present identification of C6H2N+ production using this approach allows its presence in the ISM to be inferred for the first time. This paper extends our understanding of how the dissociative ionization of benzonitrile can contribute to the abundances of radicals and other reactive species in interstellar environments.

The authors are grateful for valuable discussions with Anita Dawes at the OU. The expert technical support provided by T. Webley, K. Dewar, A. Maldonado, and their colleagues at the OU is acknowledged. The OU’s logistical and financial support is also acknowledged, including fund- ing DBD’s PhD studentship. S.E. acknowledges the British EPSRC’s support through a Life Sciences Interface Fellowship (EP/E039618/1), a Career Acceler- ation Fellowship (EP/J002577/1), and a Research Grant (EP/L002191/1). F.D.S. acknowledges the Portuguese National Funding Agency FCT-MCTES through the research grant number UID/ FIS/00068/2020 (https://doi.org/10. 54499/UIDP/00068/2020) (CEFITEC). F.D.S. is also grateful for the funding for project 21GRD02 BIOSPHERE from the European Partnership on Metrol- ogy (Funder ID: 10.13039/100019599), co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. L.M.C. acknowledges the FAPESP funding agency under process nr. 2020/04822-9. D.B.D. acknowledges the Sir John and Lady Mason Academic Trust for supporting his contributions to experiments in Lisbon. © Copyright 2025 Elsevier B.V., All rights reserved.