Multireference configuration interaction study of the predissociation of C via its FΠ state.

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.