Abstract Dicyanoacetylene cation, C4N2+, is a species of astrochemical interest. The vibronic spectrum of the ground electronic state of the C4N2+ ( X 2 Π u ) 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 (Rankovic 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 + 2 Π u ← X 1 Σ g + transition with the aim to elucidate more details about this process.

中文翻译：

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基电子态二氰基乙炔阳离子的从头算研究：振动耦合和光电离选择规则

摘要 二氰基乙炔阳离子，C4N2+，是一种具有天体化学兴趣的物质。C4N2+ ( X 2 Π u ) 基电子态的振动谱受 Renner-Teller 和自旋轨道耦合的组合影响。2011 年首次对 C4N2+ 基电子态中的振动能级进行了理论预测（Rankovic 等，2011）。最近，拉马尔等人。(2015) 报道了通过 PFI-ZEKE 光电子能谱获得的该物种的振动光谱。他们报告了实验结果与之前的理论预测之间的差异。在目前的工作中，通过使用显式相关耦合聚类方法 CCSD(T)-F12 获得的 Renner-Teller 参数解决了这些差异，其中包括足够程度的动态相关性。使用这些参数，以变分方式计算振动谱。提出了两个实验观察到的振动水平的重新分配，并实现了计算和实验观察到的水平之间的一致性。此外，将光电离选择规则应用于 X + 2 Π u ← X 1 Σ g + 跃迁，旨在阐明有关此过程的更多细节。