Determination of the Absorption Cross Section for CH3 at 216.36 nm and the Absolute Rate Constant for Methyl Radical Recombination over the Temperature Range 296-577 K.

Article abstract of DOI:10.1021/j100257a024

The absorption cross section, ?, 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 zero-time CH3 absorbance following laser flash photolysis of azomethane at 193 nm. ? 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-temperature determinations gave cross sections in good agreement < 4.12+/- 0,24)E-17 cm² (end product analysis) and (4.01+/-0,26)E-17 cm² (repeated photolysis)>.Combining these cross sections with our recent determination of k/q, where k is the recombination rate constant, gives a limiting high-pressure rate constant (k infinite) at 296 K of (6.47+/-0.20)E-11 cm³/molecule*sec which is significantly higher than the currently recommended value. k infinite decreases slightly with temperature, and it may be expressed in the form k infinite = (4.09+/-0.36)E-11 exp((137+/-33)/(T/K)) cm³/molecule*sec over the temperature range 296-577 K.The quoted errors represent 95percent confidence limits.