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Molecular modulation spectrometry has been used to measure the overall cross-reaction rate constants for the reactions of CH3 radicals with C2H5 i-C3H7 and t-C4H9 radicals at 308 K. The values of the rate constants obtained are (in cm3 mol-' s-I): CH, + C2H5, k = (28.12.8 * 1.1) x Mutual reaction rate constants and absorption cross-sections for these radicals have also been measured in the same system. The values obtained are (k in cm3 mol-' s-l and u in cm2 molecule-'): CH3 k = (20.8 f 0.8) x 10l2 and u = ( 3 0 . 8 * 0 . 8 ) ~ lo-**; C2H5, k=(10.2*00.9)x10'2 and u = ( 1 . 8 9 * 0 . 1 0 ) ~ t-C4H9, k = (4.96 f 0.20) x 10l2 and u = (7.47 * 0.24) xCombining these results with the appropriate disproportionation-combination ratios gives values for the corresponding combination and disproportionation rate constants, and cross-reaction and cross-combination ratios have been evaluated. An evaluation of the data for i-C3H7 + i-C3H7 leads to k = (6.69 f 2.14)x 1OI2 cm3 mol-' s-' being recommended as the best value for the overall rate constant in the temperature range 300-400 K. C3H7, k = (6.78k0.58) x 10l2 and u = (3.89*0.19) xReactions between like radicals have been extensively studied, not only for their intrinsic interest, but also because of their importance in combustion processes' and as reference reactions in gas kinetics studies2 However, in most complex reactions two or more different radicals are present, and in order to model such systems the rate constants for the various inter-radical reactions involved must be known.Most of the recent measurements of the rate constants for the mutual reactions of alkyl radicals have been made by means of techniques such as flash photolysis (FP)334 and molecular modulation spectrometry ( MMS),5*6 in which the concentration of radicals is monitored, usually by absorption spectroscopy. In the case of the cross-reactions between unlike radicals, however, rate constants have only been measured indirectly, in the form of rate constant ratios, by means of end-product analysis.778In this paper we present the results of an MMS study of the reactions of CH3 radicals with C2H5, i-C3H7 and t-C4H, radicals. These are the first measurements of cross-reaction rate constants in which radical concentrations have been measured directly. For the sake of consistency we have also measured rate constants for the mutual reactions of CH3, C2H5, i-C3H7 and t-C,H, radicals in the same apparatus.$ Experimental Apparatus and ProcedureThe MMS method and the details of the apparatus have been described previ~usly.~ Radicals are produced by the square-wave photolysis of a particular substrate, and are

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Rate constants for H atom attack on GeH, have been measured in pulsed photolysis experiments in the temperature range 293-473 K. H atoms were generated by the Hg-sensitized photolysis of H, and were monitored by Lyman-a absorption. The values obtained are given by k,/cm3 s-' = (0.98 f 0.13) x lo-'' exp[(-926 & 49)/7-J An evaluation of the available data on the reaction has been undertaken, and the best value for the rate constant over the temperature range 200-500 K is recommended to be k,/cm3 s-' = (1.21 f 0.10) x lo-'' exp[(-1008 f 25)/7-J