Intrinsic spectral and kinetic parameters have been measured for the ethylperoxy radical, which was formed in the gas phase by the flash photolysis of azoethane in the presence of an adequate excess of oxygen. Absolute values of the extinction coefficient €(A) were derived from complementary measurements of the yield of nitrogen and the absorbance of an equivalent concentration of ethylperoxy radicals. The absorption spectrum is broad, structureless and comparatively weak; ~(236) = 1.02 X lo3 liter mole-' cm-I a t the maximum, and the oscillator strength is 3.4 X This spectrum resembles the spectrum of the methylperoxy radical closely in form, but it is less intense; the ratio of the values of oscillator strength is 0.5. The bimolecular reactions of mutual interaction of ethylperoxy radicals are not exclusively terminating, and ethoxy and hydroperoxy radicals are formed in kinetically significant quantities. A computer program was designed to simulate the rise and fall of the concentration of each radical species, and to perform the related kinetic analysis. This program predicted that a second-order plot of the decline of the absorbance of the ethylperoxy radical during the dark period would not show a significant departure from linearity, a conclusion which was confirmed by experiment. Accordingly, the gradient of each such plot yielded a value of k ' / t ( X ) , where k' is the apparent value of the rate constant for the collective reactions of mutual interaction. This rate constant was evaluated from the product of corresponding values of k'/c(X) and €(A); individual values are independent of the wavelength of measurement, and the mean value is k' = (6.6 f 0.5) X loT liter mole-'sec-'. Further kinetic analysis yielded the corresponding absolute value: k = (6.0 f 0.6) X lo7 liter mole-' sec-l. This value fits the pattern of a relationship between rate constant and structure shown by the methylperoxy, isopropylperoxy, and tert -butylperoxy radicals. Adequate sensitivity for the characterization of the spectrum of the ethylperoxy radical was achieved by the use of a pulsed xenon arc as the monitoring light source in conjunction with a dual beam detection system with twin cells and balanced photomultipliers, and the apparatus is described in detail.

The mutual combination reactionis proposed as the rate-limiting step in the removal of C10 radicals a t moderate pressures. The third-order rate constants measured a t room temperature were kl(Ar) = 3.51 f 0.14 X lo9 lz/mo12.sec, kl(He) = 2.8 X lo9 l2/molZ.sec, and k l ( O 2 ) 0 7.9 X lo9 12/mo12-sec. There is also an independent second-order reaction2C10 -products for which hs = 8 X lo6 I/mol.sec. A new absorption spectrum has been observed in the ultraviolet and attributed to ClzOz. The extinction coefficient for C1202 has been measured a t six wavelengths, and, between 292 and 232 nm, it increases from 0.4 X lo3 to 2.9 X lo3 1/ mol-cm. In the presence of the chlorine atom scavengers OClO or ClzO, ClzOz exists in equilibrium with C10. The equilibrium constant Kel = 3.1 f 0.1 X lo6 l/mol a t 298 K, and, with AS? estimated to be -133 f 11 J/K-mol, AH: = -69 f 3 kJ/mol and AHP(Clz0z) = 136 f 3 kJ/mol.

The ultraviolet absorption spectra of the acetyl and acetylperoxy radicals have been Characterized in the range 195-280 nm. The acetyl radical was generated by the flash phbtolysis of Clz in the presence of CH3CH0 and was converted to the acetylperoxy radical in the presence of excess O2 The extinction coefficient of the acetylperoxy radical was measured to be 2300 L/mol cm at the maximum at 207 nm and the rate constant for the reactionwas evaluated to be k, = (4.8 2 0.8) x lo9 L/mol s

The kinetics of the reaction CH3 + 0 2 (+M) -+ CH302 (+MI have been studied, using the technique of flash photolysis and kinetic spectroscopy to follow the methyl radical concentration. The order of the reaction lies between 2 and 3 throughout the range of pressure from 25 to 380 torr at 22"C, and the results are consistent with a single reaction sequence:The limiting values of the third-order rate coefficients at low pressures are (3.6 f 0.3) X 10" 1. mole-2 sec-' when M is neopentane, and (0.94 f 0.03) X 10" L 2 mole-2 sec-' when M is nitrogen. The limiting value of the second-order rate coefficient at high pressures is (3.1 f 0.3) X lo8 1. mole-' sec-I. The rate constant for the independent second-order reaction CH3 + 0 2 -+ CH20 + OH is shown to be not much greater than 2 X lo5 1. mole-' sec-I, so that this reaction does not complete significantly with the combination reaction.This new interpretation is contrary to currently accepted views.

The rate constant for the reaction CH302 + NO2 -(products) has been measured directly by flash photolysis and kinetic spectroscopy. At room temperature and a t total pressures between 53 and 580 Torr, k:, = (9.2 f 0.4) X lo8 liter/mole sec so that the rate of formation of the probable primary product peroxymethyl nitrate (CH302N02) may be significant in urban atmospheres.