Vacuum Ultraviolet Phototodissociation of CS and C₂

Photodissociation is a major destruction pathway for small molecules in a number of astrophysical environments, such as photon-dominated regions, the diffuse interstellar medium, and protoplanetary disks. Accurate wavelength-dependent photodissociation cross sections, including both direct photodissociation and predissociation, are a critical input for astrochemical models. For many stable diatomic molecules (e.g., CO), reliable laboratory measurements have been made over wide wavelength ranges. However, measuring absolute cross sections for reactive molecules such as CS and C₂, both of which are abundant in space, is a major challenge. In both of these molecules, predissociation plays a major role in the vacuum ultraviolet region (100-300 nm), yet few experimental or theoretical studies have targeted electronic states in this energy range. We have performed ab initio calculations using the CASSCF/MRCI+Q method to characterize excited electronic states of CS and C₂ relevant for predissociation. For CS, we focus especially on the B ¹Σ⁺ and C ¹Σ⁺ Rydberg states, both of which are accessible via allowed transitions from the ground X ¹Σ⁺ state (B-X at 154 nm, and C-X at 140 nm). Using a coupled channel model with a diabatic treatment of several ¹Σ⁺, ³Π, and ³Σ^- states, we compute new ab initio photodissociation cross sections that result in a threefold higher photodissociation rate under the standard interstellar radiation field compared with the cross sections available in the Leiden photodissociation database. For C₂, we target the Herzberg F ¹Π_u - X ¹Σ_g⁺ band at 132 nm, which has been observed in Hubble spectra, deriving ab initio spectroscopic constants and oscillator strengths. Ongoing progress for treating the predissociation process as well as new experimental measurements will be discussed.