J . Phys. Chem. 1985, 89, 2268-2274
2268
Determination of the Absorptlon Cross Section for CH, at 216.36 nm and the Absolute Rate Constant for Methyl Radical Recomblnation over the Temperature Range 296-577 K Martyn T. Macpherson, Michael J. Pilling,* and Martin J. C. Smith Physical Chemistry Laboratory, Oxford University, Oxford, OX1 3Q2,U.K. (Received: December 18, 1984)
The absorption cross section, u, for the CH3 transition at 216.36 nm has been measured for a 0.60-nm band-pass. At 298 K, the cross section was determined by comparing the ethane yield, as measured by gas chromatography, with the zero-time CH3 absorbance following laser flash photolysis of azomethane at 193 nm. u was also measured over the temperature range 298-537 K by a repetitive photolysis method in which a known initial pressure of azomethane was repeatedly photolyzed with an ArF laser and the zero-time absorbance determined following each laser pulse. The two room-temperaturedeterminations gave cross sections in good agreement [(4.12 f 0.24) X lo-'' cmz (end product analysis) and (4.01 f 0.26) X 1O-I' cm2 (repeated photolysis)]. Combining these cross sections with our recent determination of k / u , where k is the recombination rate constant, gives a limiting high-pressure rate constant ( k , ) at 296 K of (6.47 h 0.20) X lo-" cm3 molecule-' s-l which is significantly higher than the currently recommended value. k , decreases slightly with temperature, and it may be expressed in the form k , = (4.09 h 0.36) X lo-" exp((137 f 33)/(T/K)) cm3 molecule-' s-' over the temperature range 296-577 K. The quoted errors represent 95% confidence limits.
perature range if the T dependence of the recombination rate constant is to be determined. Our recent measurements* suggest that k , decreases slightly with T, although this conclusion depends sensitively on a( T). In this paper, we describe the results of two independent methods for determining the absorption cross section, which provide values of u with an uncertainty of -*5% over the temperature range 298-537 K.
Introduction
The recombination of methyl radicals is of importance as a terminating reaction in pyrolysis and combustion,' as a standard reaction in steady-state competitive techniques for determining rate constants for CH3-molecule reactions,* and as a testing ground for unimolecular rate t h e ~ r y . ~ ,Most ~ experimental measurements refer to room temperature and pressures close to the high-pressure limit, although some measurements have been made in the falloff region at higher temperat~res.~'These results have been critically evaluated by Baulch and Duxbury.l More recently, we have studied the reaction in the falloff region over the temperature range 296-577 K.' Most experimental measurements of CH3 recombination have emp!oy-d optical detection of CH3, generally at -216 nm, using the B-X transition. Thus, the quantity measured directly is k / u , where k is the recombination rate constant for CH3
+ CH3
-
Experimental Section
The apparatus and methods of analyzing decay curves have been described previously.'0 The methyl radical was monitored at 216.36 nm with a 0.60-nm band-pass (full width at half-maximum, with a triangular slit function). The monochromator was calibrated against the 213.86-nm line of a low-pressure zinc lamp. Variations in the recorded values of k / u , which were small (
