Cl₂ Elimination in 248 nm Photolysis of (COCl)₂ Probed with Cavity Ring-Down Absorption Spectroscopy

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)₂ at 248 nm obtaining a primary Cl₂ elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl₂ molecular channel is obtained to be 0.8 ± 0.4 initiated from the X̃ ¹A_g ground state surface (COCl)₂ via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)₂ + hν → 2CO + Cl₂) and secondary reactions (dominated by Cl + COCl → Cl₂ + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl₂ is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl₂ yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl₂ dissociation channel eliminated from (COCl)₂ is observed for the first time.