Multireference configuration interaction study of the predissociation of C-2 via its F-1?(u) state

Photodissociation is one of the main destruction pathways for dicarbon (C-2) 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-2 features a strong predissociative F-1 Pi(u )- X-1 sigma(+)(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 F-1 Pi(u ) state are limited. In this work, potential energy curves of excited electronic states of C-2 are calculated with the aim of describing the predissociative nature of the F-1 Pi(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 F-1 Pi(u ) state are calculated by a coupled-channel model. The total integrated cross section through the F-1 Pi(u ) v = 0 and v = 1 bands is 1.198 x 10(-13) cm(2) cm(-1), giving rise to a photodissociation rate of 5.02 x 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 2(1)sigma(+)(u) state that should be detectable via a strong 2(1)sigma(+)(u)-X-1 sigma(+)(g) band around 116 nm. Published under an exclusive license by AIP Publishing.