Vibronic structure of the cyanobutadiyne cation. I. VUV photoionization study of HC5N.

We report the vacuum-ultraviolet threshold-photoelectron spectrum of HC₅N recorded over a wide spectral range, from 84 000 to 120 000 cm⁻¹, with a 120 cm⁻¹ spectral resolution, better than what was achieved in previous photoelectron studies, and with mass selectivity. The adiabatic ionization potential of cyanobutadiyne is measured at 85 366 (±40) cm⁻¹. Assignment of the vibrational bands of the four lowest electronic states X⁺²Π, A⁺²Π, B⁺²Σ⁺, and C⁺²Π are performed, supported by high level ab initio calculations which are fully detailed in Paper II [B. Gans et al., J. Chem. Phys. 150, 244303 (2019)] and by Franck-Condon simulations. Only vibrational stretching modes are observed in the threshold-photoelectron spectra. The ground state of HC₅N⁺ exhibits a vibrational progression in the ν₂ stretching mode involving mainly the elongation of the C≡C triple bonds, whereas the A⁺ and C⁺ excited electronic states show a progression in the stretching mode mainly associated with the elongation of the C≡N bond, i.e., ν₄ and ν₃, respectively. The B⁺ state appears almost as a vibrationless structure in close vicinity to the A⁺ state. [ABSTRACT FROM AUTHOR]