Experimental and ab initio characterization of HCN vibronic structure. I. Synchrotron-based threshold photo-electron spectroscopy.

Threshold-photoionization spectroscopy of cyanoacetylene (HCN) and its N isotopologue has been investigated in the vacuum-ultraviolet range with a synchrotron-based experiment allowing to record threshold-photoelectron spectrum and photoion yield over a large energy range (from 88 500 to 177 500 cm, i.e., from 11 to 22 eV). Adiabatic ionization energies towards the three lowest electronic states XΠ, A Σ+2, and B Π2 are derived from the threshold-photoelectron spectrum. A detailed description of the vibrational structure of these states is proposed leading to the determination of the vibrational frequencies for most modes. The vibrational assignments and the discussion about the electronic structure are supported by multireference ab initio calculations (CASPT2, MRCI). Unprecedented structures are resolved and tentatively assigned in the region of the B← X transition. Exploratory calculations highlight the complexity of the electronic landscape of the cation up to approximately 10 eV above its ground state.