Ab initio investigation of dicyanoacetylene cation in the ground electronic state: Vibronic coupling and photoionization selection rules

Abstract

Dicyanoacetylene cation, C4N2+, is a species of astrochemical interest. The vibronic spectrum of the ground electronic state of the C4N2+ (X2Πu) is affected by a combination of the Renner–Teller and spin-orbit coupling. The first theoretical prediction of the vibronic energy levels in the ground electronic state of C4N2+ was carried out in 2011 (Ranković et al., 2011). Recently, Lamarre et al. (2015) reported the vibronic spectrum of this species obtained by the PFI-ZEKE photoelectron spectroscopy. They reported discrepancies between the experimental results and the previous theoretical predictions. In the present work, those discrepancies were resolved by using Renner-Teller parameters obtained by the explicitly correlated coupled-cluster method, CCSD(T)-F12, which includes dynamical correlation to a sufficient extent. Using these parameters, the vibronic spectrum was calculated in a variational manner. Reassignment of two experimentally observed vibronic levels is proposed and agreement between the calculated and experimentally observed levels was achieved. Furthermore, the photoionization selection rules were applied to the X+2Πu←X1Σg+ transition with the aim to elucidate more details about this process.