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Photodissociation is one of the main destruction pathways for dicarbon(C$_{2}$) in astronomical environments such as diffuse interstellar clouds, yetthe accuracy of modern astrochemical models is limited by a lack of accuratephotodissociation cross sections in the vacuum ultraviolet range. C$_{2}$features a strong predissociative $F\,^1\Pi_u - X\,^1\Sigma_g^+$ electronictransition near 130 nm originally measured in 1969; however, no experimentalstudies of this transition have been carried out since, and theoretical studiesof the $F\,^1\Pi_u$ state are limited. In this work, potential energy curves ofexcited electronic states of C$_{2}$ are calculated with the aim of describingthe predissociative nature of the $F\,^1\Pi_u$ state and providing new abinitio photodissociation cross sections for astrochemical applications.Accurate electronic calculations of 56 singlet, triplet, and quintet states arecarried out at the DW-SA-CASSCF/MRCI+Q level of theory with a CAS(8,12) activespace and the aug-cc-pV5Z basis set augmented with additional diffusefunctions. Photodissociation cross sections arising from the vibronic groundstate to the $F\,^1\Pi_u$ state are calculated by a coupled-channel model. Thetotal integrated cross section through the $F\,^1\Pi_u$ $v=0$ and $v=1$ bandsis 1.198$\times$10$^{-13} $cm$^2$cm$^{-1}$, giving rise to a photodissociationrate of 5.02$\times$10$^{-10}$ s$^{-1}$ under the standard interstellarradiation field, much larger than the rate in the Leiden photodissociationdatabase. In addition, we report a new $2\,^1\Sigma_u^+$ state that should bedetectable via a strong $2\,^1\Sigma_u^+-X\,^1\Sigma_g^+$ band around 116 nm.

Multireference configuration interaction study of the predissociation of C$_{2}$ via its $F\,^1\Pi_u$ state