Threshold photoelectron spectroscopy and dissociative photoionization of benzonitrile

The threshold photoionization and dissociative ionization of benzonitrile (C₆H₅CN) were studied using double imaging photoelectron photoion coincidence (i²PEPICO) spectroscopy at the Vacuum Ultraviolet (VUV) beamline of the Swiss Light Source (SLS). The threshold photoelectron spectrum was recorded from 9.6 to 12.7 eV and Franck-Condon simulations of ionization into the ionic ground state, X̃⁺, as well as the B̃⁺ and C̃⁺ states were performed to assign the observed vibronic structures. The adiabatic ionization energies of the X̃⁺, B̃⁺ and C̃⁺ states are determined to be (9.72 ± 0.02), (11.85 ± 0.03) and, tentatively, (12.07 ± 0.04) eV, respectively. Threshold ionization mass spectra were recorded from 13.75 to 19.75 eV and the breakdown diagram was constructed by plotting the fractional abundances of the parent ion and ionic dissociation products as a function of photon energy. The seven lowest energy dissociative photoionization channels of benzonitrile were found to yield CN˙ + c-C₆H₅⁺, HCN + C₆H₄˙⁺, C₂H₄ + HC₅N˙⁺, HC₃N + C₄H₄˙⁺, H₂C₃N˙ + C₄H₃⁺, CH₂CHCN + C₄H₂˙⁺ and H₂C₄N˙ + c-C₃H₃⁺. HCN loss from the benzonitrile cation is the dominant dissociation channel from the dissociation onset of up to 18.1 eV and CH₂CHCN loss becomes dominant from 18.1 eV and up. We present extensive potential energy surface calculations on the C₆H₅CN˙⁺ surface to rationalize the detected products. The breakdown diagram and time-of-flight mass spectra are fitted using a Rice-Ramsperger-Kassel-Marcus statistical model. Anchoring the fit to the CBS-QB3 result (3.42 eV) for the barrier to HCN loss, we obtained experimental dissociation barriers for the products of 4.30 eV (CN loss), 5.53 eV (C₂H₄ loss), 4.33 eV (HC₃N loss), 5.15 eV (H₂C₃N loss), 4.93 eV (CH₂CHCN loss) and 4.41 eV (H₂C₄N loss). We compare our work to studies of the electron-induced dissociative ionization of benzonitrile and isoelectronic phenylacetylene (C₈H₆), as well as the VUV-induced dissociation of protonated benzonitrile (C₆H₅CNH⁺). Also, we discuss the potential role of barrierless association reactions found for some of the identified fragments as a source of benzonitrile(˙⁺) in interstellar chemistry and in Titan's atmosphere.