Cl2O photochemistry: Ultraviolet/vis absorption spectrum temperature dependence and O(3P) quantum yield at 193 and 248 nm

The photochemistry of Cl2O (dichlorine monoxide) was studied using measurements of its UV/vis absorption spectrum temperature dependence and the O(3P) atom quantum yield, ΦCl2OO(λ), in its photolysis at 193 and 248 nm. The Cl2O UV/vis absorption spectrum was measured over the temperature range 201-296 K between 200 and 500 nm using diode array spectroscopy. Cl2O absorption cross sections, σCl2O(λ,T), at temperatures <296 K were determined relative to its well established room temperature values. A wavelength and temperature dependent parameterization of the Cl2O spectrum using the sum of six Gaussian functions, which empirically represent transitions from the ground 1A1 electronic state to excited states, is presented. The Gaussian functions are found to correlate well with published theoretically calculated vertical excitation energies. O(3P) quantum yields in the photolysis of Cl2O at 193 and 248 nm were measured using pulsed laser photolysis combined with atomic resonance fluorescence detection of O(3P) atoms. O(3P) quantum yields were measured to be 0.85 ± 0.15 for 193 nm photolysis at 296 K and 0.20 ± 0.03 at 248 nm, which was also found to be independent of temperature (220-352 K) and pressure (17 and 28 Torr, N2). The quoted uncertainties are at the 2σ (95% confidence) level and include estimated systematic errors. ClO radical temporal profiles obtained following the photolysis of Cl2O at 248 nm, as reported previously in Feierabend et al.[J. Phys. Chem. A 114, 12052, (2010)], were interpreted to establish a <5% upper-limit for the O + Cl2 photodissociation channel, which indicates that O(3P) is primarily formed in the three-body, O + 2Cl, photodissociation channel at 248 nm. The analysis also indirectly provided a Cl atom quantum yield of 1.2 ± 0.1 at 248 nm. The results from this work are compared with previous studies where possible.