Vibronic structure of the cyanobutadiyne cation: I-VUV photoionization study of HC 5 N

International audienceWe report the vacuum-ultraviolet threshold-photoelectron spectrum of HCN recorded over a wide spectral range, from 84 000 to 120 000 cm, with a 120 cm 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. Assignment of the vibrational bands of the four lowest electronic states XΠ, AΠ, BΣ, and CΠ 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 HCN exhibits a vibrational progression in the ν 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., ν and ν, respectively. The B state appears almost as a vibrationless structure in close vicinity to the A state