Probing the Photodissociation Mechanism of Oxalyl Chloride by Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy is an ultra-sensitive absorption spectroscopy technique. It is based on measurement of the decay rate of a pulsed laser light trapped in an optical cavity with a pair of highly-reflective mirrors. The plot of decay rate as a function of laser wavelength gives the absorption spectrum. Using CRDS, we detect the Cl_2 B^3 Π_(0+u) ← X^1 Σ_(g+) absorption transition from the molecular elimination channel following 248 nm photolysis of oxalyl chloride. Energy-dependence experiments show that Cl_2 elimination is a one-photon process. Pressure-dependence experiments show that the reaction is first-order overall. Comparing the experimental spectrum with the simulated spectrum, we can obtain the branching ratio of Cl_2 vibrational levels, which is 1 : (0.12 ± 0.03) : (0.011 ± 0.003). The branching ratio corresponds to a vibrational temperature of 360 ± 60 K, indicating the Cl_2 formed is vibrationally hot. Using a modified UV-Vis spectrophotometer, we measure the absorption cross-section of oxalyl chloride at 248 nm to be 2.78 × 10-19 cm^2, and further calculate the quantum yield of Cl_2 formation to be 0.8 ± 0.4. Kinetic modelling shows that secondary reactions could contribute to 17~27 % of the Cl_2 signal detected. At high pressures, excited oxalyl chloride is quenched because of collisions, which causes the quantum yield of Cl_2 to decrease. The self-quenching effect is also supported by laser-induced fluorescence (LIF) measurements.