Multireference configuration interaction study of the predissociation of C₂ via its F¹Π_u state

Photodissociation is one of the main destruction pathways for dicarbon (C₂) 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. C₂ features a strong predissociative F¹Π_u-X¹Σ_g ⁺ 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 F¹Π_u state are limited. In this work, potential energy curves of excited electronic states of C₂ are calculated with the aim of describing the predissociative nature of the F¹Π_u 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 F¹Π_u state are calculated by a coupled-channel model. The total integrated cross section through the F¹Π_u v = 0 and v = 1 bands is 1.198 × 10^-13 cm² cm^-1, giving rise to a photodissociation rate of 5.02 × 10^-10 s^-1 under the standard interstellar radiation field, much larger than the rate in the Leiden photodissociation database. In addition, we report a new 2¹Σ_u ⁺ state that should be detectable via a strong 2¹Σ_u ⁺-X¹Σ_g ⁺ band around 116 nm.