The photolysis of chlorine peroxide (ClOOCl) is understood to be a key step in the destruction of polar stratospheric ozone. This study generated and purified ClOOCl in a novel fashion, which resulted in spectra with low impurity levels and high peak absorbances. The ClOOCl was generated by laser photolysis of Cl2 in the presence of ozone, or by photolysis of ozone in the presence of CF2Cl2. The product ClOOCl was collected, along with small amounts of impurities, in a trap at about -125 degrees C. Gas-phase ultraviolet spectra were recorded using a long path cell and spectrograph/diode array detector as the trap was slowly warmed. The spectrum of ClOOCl could be fit with two Gaussian-like expressions, corresponding to two different electronic transitions, having similar energies but different widths. The energies and band strengths of these two transitions compare favorably with previous ab initio calculations. The cross sections of ClOOCl at wavelengths longer than 300 nm are significantly lower than all previous measurements or estimates. These low cross sections in the photolytically active region of the solar spectrum result in a rate of photolysis of ClOOCl in the stratosphere that is much lower than currently recommended. For conditions representative of the polar vortex (solar zenith angle of 86 degrees, 20 km altitude, and O3 and temperature profiles measured in March 2000) calculated photolysis rates are a factor of 6 lower than the current JPL/NASA recommendation. This large discrepancy calls into question the completeness of present atmospheric models of polar ozone depletion.

The kinetics and mechanism of the C10 + C10 reaction and the thermal decomposition of ClOOCl were studied using the flash photolysis/long path ultraviolet absorption technique. Pressure and temperature dependences were determined for the rate coefficients for the bimolecular and termolecular reaction channels and for the thermal decomposition of ClOOCl. In order to determine channel-specific rate coefficients and to minimize complications associated with secondary chemistry, the reaction was studied over wide ranges of initial reactant stoichiometry and temperature. The rate coefficient for the termolecular association channel in the lowpressure limit, C10 + C10 (+M) -ClOOCl (+M) ( l ) , with N2 as a third body was measured over the temperaturerange 195-390Kandresultedinkl,~,(T) =(1.22*0.15) X 1&33exp{(833 f 34)/TJ~m~molecule-~ s-1 ( f 2 a error bounds). The 300 K rate coefficient for reaction 1 was measured for a number of bath gases.The results are k l ,~ (X10-32 cm6 molecule-2 s-l) = 0.99 f 0.05, 1.24 f 0.09, 1.71 f 0.06, 2.00 f 0.27, 2.60 f 0.17, 3.15 f 0.14, and 6.'/ f 3.6 for He, 0 2 , Ar, N2, CF4, SF6, and C4, respectively. The effective collision efficiency for M = C12 is very large and is likely due to a chaperone mechanism. Below 250 K, the reaction was in the falloff regime between second-and third-order kinetics. From the falloff data, the rate constant in the high-pressure limit, k,3w, was estimated to be (6 f 2) X cm3 molecule-' s-'. The Arrhenius expressions for the three bimolecular channels, C10 + C10 -Cl2 + 0 2 (2), ClOO + C1(3), and OClO + C1 (4), over the temperature range 260-390 K are kz(7') = (1.01 f 0.12) X exp(-(1590 f lOO)/TJ cm3 molecule-' s-1, k3(7') = (2.98 f 0.68) X 10-11 exp{-(2450 f 330)/Tj cm3 molecule-' s-l, and k4(T) = (3.50 f 0.31) X 10-13 exp{-(1370 f 150)/Tj cm3 molecule-' s-l. These expressions lead to a value of (1.64 f 0.35) X lO-14cm3 molecule-1 s-1 for the overall bimolecular rate constant (k2 + k3 + k4) at 298 K. The rate coefficient expression for ClOOCl thermal decomposition was determined to be k-l( 7') = (9.81 f 1.32) X exp{-(7980 f 320)/Tj cm3 molecule-' s-1 over the range 260-310 K. From a Third Law analysis using equilibrium constants derived from measured values of kl and k-1, the enthalpy of formation (AHOr(298)) of ClOOCl was determined to be 30.5 f 0.7 kcal mol-'. The equilibrium constant expression from this analysis is K,(T) = (1.24 f 0.18) X 10-27 exp((8820 f 440)/Tj cm3 molecule-'. From the observed activation energy for reaction 4 and the literature activation energy for reaction -4, the OClO enthalpy of formation was calculated to be 22.6 f 0.3 kcal mol-'.

The kinetics of the dimerization of ClO radicals, ClO + ClO + M f 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 r X ) 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.

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Complex formation in the interaction of HO2 with H2O was studied using Fourier transform infrared absorption spectroscopy. Studies were carried out between 230 and 298 K in the presence of excess H2O. On the basis of measurements of HO2 disappearance, equilibrium constants (K c) for the formation of the complex were determined to be 1.7 × 10-16, 4.1 × 10-17, and ≤4 × 10-18 cm3 molecule-1 at 230, 250, and 298 K, respectively. The enthalpy for the reaction forming the complex was determined to be −36 ± 16 kJ mol-1. This is in good agreement with previous estimates made from kinetic measurements and theoretical calculations.