Xu Zhongxing, Federman S R, Jackson William M, Ng Cheuk-Yiu, Wang Lee-Ping, Crabtree Kyle N
Department of Chemistry, University of California, Davis, Davis, California 95616, USA.
Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606, USA.
J Chem Phys. 2022 Jul 14;157(2):024302. doi: 10.1063/5.0097451.
Photodissociation is one of the main destruction pathways for dicarbon (C) in astronomical environments, such as diffuse interstellar clouds, yet the accuracy of modern astrochemical models is limited by a lack of accurate photodissociation cross sections in the vacuum ultraviolet range. C features a strong predissociative FΠ-XΣ electronic transition near 130 nm originally measured in 1969; however, no experimental studies of this transition have been carried out since, and theoretical studies of the FΠ state are limited. In this work, potential energy curves of excited electronic states of C are calculated with the aim of describing the predissociative nature of the FΠ state and providing new ab initio photodissociation cross sections for astrochemical applications. Accurate electronic calculations of 56 singlet, triplet, and quintet states are carried out at the DW-SA-CASSCF/MRCI+Q level of theory with a CAS(8,12) active space and the aug-cc-pV5Z basis set augmented with additional diffuse functions. Photodissociation cross sections arising from the vibronic ground state to the FΠ state are calculated by a coupled-channel model. The total integrated cross section through the FΠ v = 0 and v = 1 bands is 1.198 × 10 cm cm, giving rise to a photodissociation rate of 5.02 × 10 s under the standard interstellar radiation field, much larger than the rate in the Leiden photodissociation database. In addition, we report a new 2Σ state that should be detectable via a strong 2Σ -XΣ band around 116 nm.
光解离是天文环境（如弥漫星际云）中二碳（C₂）的主要破坏途径之一，但现代天体化学模型的准确性受到真空紫外范围内缺乏精确光解离截面的限制。C₂在130nm附近有一个强预解离的FΠ-XΣ电子跃迁，最初于1969年测得；然而，此后尚未对该跃迁进行实验研究，并且对FΠ态的理论研究也很有限。在这项工作中，计算了C₂激发电子态的势能曲线，目的是描述FΠ态的预解离性质，并为天体化学应用提供新的从头算光解离截面。在DW-SA-CASSCF/MRCI+Q理论水平下，使用CAS(8,12)活性空间和aug-cc-pV5Z基组并添加额外的漫射函数，对56个单重态、三重态和五重态进行了精确的电子计算。通过耦合通道模型计算了从振转基态到FΠ态的光解离截面。通过FΠ v = 0和v = 1能带的总积分截面为1.198×10⁻¹⁸ cm²，在标准星际辐射场下产生的光解离速率为5.02×10⁻⁷ s⁻¹，远大于莱顿光解离数据库中的速率。此外，我们报告了一个新的²Σ⁺态，应该可以通过116nm附近的强²Σ⁺-XΣ⁺能带检测到。
