Kinetics of the ClO Self-Reaction and 210 nm Absorption Cross Section of the ClO Dimer

The kinetics of the dimerization of ClO radicals, ClO + ClO + M → Cl 2 O 2 + M (1a), and the 210 nm absorption cross sections of the ClO dimer have been studied using the technique of flash photolysis with UV absorption spectroscopy, over the temperature range 183−245 K and pressure range 25−700 Torr. ClO radicals were generated following the photolysis of Cl 2 /Cl 2 O/N 2 mixtures and were quantified via their differential absorption between the peak of the (12,0) band of the (Ã ← X̃) transition at 275.2 nm and the adjacent minimum to higher wavelengths, while Cl 2 O 2 formation was simultaneously monitored at 210 nm. ClO differential absorption cross sections were measured under identical conditions to the kinetic experiments by four separate calibration schemes. The rate coefficient measured at the lower temperatures studied ( T < 200 K) was found to be up to 40% faster than extrapolation of previous results would suggest, with the limiting low- and high-pressure rate coefficients for reaction 1 in nitrogen determined to be k 0 = (1.59 ± 0.60) × 10 -32 × ( T /300) -4.50±0.98 molecules -2 cm 6 s -1 and k ∞ = (1.36 ± 0.22) × 10 -12 × ( T /300) -3.09±0.40 molecules -1 cm 3 s -1 respectively, obtained with F c = 0.6. The corresponding value for k 0 in air is k 0(atm) = (1.49 ± 0.56) × 10 -32 × ( T /300) -4.50±0.98 molecules -2 cm 6 s -1 . The 210 nm absorption cross section of Cl 2 O 2 was measured following time-resolved monitoring of its formation via ClO radical association, and a mean value of (2.94 ± 0.86) × 10 -18 molecule -1 cm 2 obtained, temperature independent (to within ± 15%) between 183 and 245 K. Errors are combined systematic uncertainties and 2 standard deviations statistical variation.