Geometry of C2N2 + and HCN+ from low-energy photoelectron spectroscopy

Quantitative Franck-Condon calculations are applied to the first two band systems of the low energy photoelectron spectrum of dicyanogen (C₂N₂) and to the first band system of hydrogen cyanide. Estimates are made of the bond length changes from the ground electronic state of the molecule to various states of C₂N₂ ⁺ and to the ground state of HCN⁺. These changes are consistent with expectations based on simple molecular orbital theory.

It is shown that there is sufficient intensity in the perturbed first system of HCN, and also that of DCN, to account for an overlapping second system due to the removal of an electron from the 5σ orbital.