The ultraviolet spectra of the alkyl peroxy radicals, CH302 and C(CH&02, have been obtained using flash photolysis with photoelectric recording. They are in good agreement with those found using molecular modulation spectroscopy. The overall rates of the '' mutual *' reactions : and 2CH302 --f products(3) 2C(CH&02 + products (4) also agree very well with rates found previously.Two other reactions of importance in low temperature oxidation have been studied. A lower limit of cm3 molecule-I s-I was found for the rate constants of the overall reactions of CH302 and of C(CH&02 with NO. The rate constant for the reaction between OH and t-butyl hydroperoxide was found to be (3.0+ 0.8) x cm3 molecule-' s-l at room temperature.This study forms part of investigations at both Cambridge and Thornton into the kinetics of reactions which are important in low temperature combustion and the atmospheric oxidation of hydrocarbons. Using molecular modulation spectroscopy (m.m.s.),l we have previously obtained spectra and monitored the kinetics of alkyl peroxy radicals in the gas phase.2* Virtually identical spectra have been obtained from the liquid phase.4* that spectra obtained by modulated photolysis could be distorted by HOz absorption, particularly in experiments on methyl peroxy radicals, from which H02 is formed by subsequent reactions. We have ourselves also been concerned that products of the reactions between peroxy radicals could build up and affect the measurements in the modulation spectrometer. Therefore, to check our earlier results, we decided to generate peroxy radicals by flash photolysis. During the course of this work, the results of a similar study of methyl peroxy radicals have been published,' and we have also learnt of a third independent study.8 The results of our own work on methyl peroxy radicals are in essential agreement with those from these other investigations ; in addition, our experiments have provided the first data on t-butyl peroxy radicals generated by flash photolysis.

Using the technique of molecular modulation spectrometry, we have found transient ultraviolet absorptions which we assign to ethyl, isopropyl and t-butyl radicals. The spectra are broad continua with maxima near 250, 233 and 230nm, respectively. From the rate constants of their mutual reactions, measured by monitoring the radicals in the ultraviolet, and the disproportionation/ recombination ratios found from separate experiments, the rates of recombination are measured at room temperature to be : ethyl, (1.3 k 0.3) x 10-l' cm3 molecule-' s-l ; isopropyl, (8.3 2 2.0) x 10-l2 cm3 molecule-' s-' ; t-butyl, (4.0+ 1.0) x cm3 molecule-' s-l. The t-butyl rate falls by a factor of 2 as the temperature is increased to 423 K. These results are compared with other recent gas-and liquid-phase determinations, and we conclude that the currently accepted thermochemistry of larger alkyl radicals and our results are incompatible.

Using the technique of molecular modulation spectrometry, we have measured directly the rate constants of several reactions involved in the oxidation of methyl radicals at room temperature:(1)kl is in the fall-off pressure regime at our experimental pressures (20-760 torr) where the order lies between second and third and we obtain an estimate for the second-order limit of (1.2 f 0.6) X cm3/molec . sec, together with third-order rate constants of CH3 + CH302 --t 2CHaO k3 = (6.0 f 1.0) X 10-11 cm3/molec -sec.

The negative ions formed in NzO and NzO+O2 mixtures have been studied in the gas phase using a drift tube and mass filter. Gas pressures were in the Torr range, and reduced fields were varied between lo-'' and V cm2 molecule-'. The observed ion spectrum was found to be governed by the following reactions, with their associated thermal rate constants : e+N20+N2+0-, 0-+N20-+NO-+N0,NO-+N20-+N20+NO+e, kll = (6.0+ 1.0) x 10-l2 cm3 molecule-' s-'The rates of reactions (5), (6) and (1 1) were weak functions of reduced field. In no experiment was any evidence found for the existence of a long-lived N20ion.Nitrous oxide is used widely in radiation chemistry as an electron scavenger.I Its action has been interpreted in terms of the reaction :

The attachment coefficient and drift velocity of electrons in nitric oxide have been measured in a pulsed drift tube, over a reduced field range between 6 x 10-l8 and 2 x 10-l6 V cmz molecule-', at gas densities of ca. 3 x lo+'* molecule ~m -~ and ambient temperatures between 293 and 493 K. The resultant attachment frequency, described as second order in nitric oxide pressure at room temperature, reached a peak value of 12 x cm6 molecule-z s-' at 0.2 eV electron energy, falling off at higher and lower energies. The thermal rate found by extrapolation is 8 1 2 x cm6 s-'. At higher temperatures the measured coefficient fell, indicating the occurrence of detachment reactions. The detachment rate was measured in separate experiments using a d.c. drift tube and mass filter and found to be 5 & 1 x cm3 molecule-' s-l, for near thermal ions. Comparison of the measured attachment and detachment rates with the statistical mechanical formula for the equilibrium constant yields a value of 0.026&0.02 eV for the electron affinity of nitric oxide.The only stable ion formed in NO was mass 46, NO;, formed by subsequent ion molecule reactions from the initial product NO-. In NO+Oz mixtures the rate of the following reaction was measured :NO;+NO --f NO;+NO,, k = 3 x cm3 molecule s-'. From equilibrium considerations, it is deduced using the value of this rate constant that the electron affinity of NO3 exceeds that of NOz by at least 1.13 eV.