Experimental and ab initio characterization of HCN vibronic structure. II. High-resolution VUV PFI-ZEKE spectroscopy.

Vacuum-ultraviolet pulsed-field-ionization zero-kinetic-energy photoelectron spectra of XΠ2←XΣ+1 and BΠ2←XΣ+1 transitions of the HCN and HCN isotopologues of cyanoacetylene have been recorded. The resolution of the photoelectron spectra allowed us to resolve the vibrational structures and the spin-orbit splittings in the cation. Accurate values of the adiabatic ionization potentials of the two isotopologues (E/hc(HCN)=93 909(2) cm and E/hc(HCN)=93 912(2) cm), the vibrational frequencies of the ν, ν, and ν vibrational modes, and the spin-orbit coupling constant (A = -44(2) cm) of the XΠ2 cationic ground state have been derived from the measurements. Using ab initio calculations, the unexpected structure of the BΠ2←XΣ+1 transition is tentatively attributed to a conical intersection between the A and B electronic states of the cation.