Observation of CH4 (v2=1 or v4=1) in the reaction Cl+CH4 with time-resolved Fourier-transform infrared absorption spectroscopy

The reaction Cl(P-2) +CH4 was initiated on laser irradiation of a flowing mixture containing Cl-2, CH4, and Ar at 355 nm; reaction products were monitored with a step-scan time-resolved Fourier-transform absorption spectrometer coupled with a multipass absorption cell. Not only loss of CH4 but also production of HCl, CH3Cl, highly rotationally excited CH4 [designated as CH4(J*)], and vibrationally excited CH4 (v(2)=1 or v(4)=1), designated as CH4(v*), was observed after laser irradiation. Absorption lines of CH4(J*) and CH4(v*) are assigned according to published spectral parameters. Rates of formation and decay-of CH4(v*) are derived on fitting observed temporal profiles with a simple kinetic model. A bimolecular rate coefficient for formation of CH4(v*) is determined to be (1.1+/-0.2) x 10(-14) cm(3) molecule(-1) s(-1), nearly identical to that reported for the reaction Cl+CH4. Experimental evidence indicates that the reaction Cl+CH4 is rate determining to formation of CH4(v*). CH4(v*) is likely produced through energy transfer from vibrationally excited CH3Cl that is produced via secondary reactions. A rate coefficient for relaxation of CH4* by collision with Ar is determined to be (2.2+/-0.1) x 10(-15) cm(3) molecule(-1) s(-1), consistent with previous results. The proportion of CH4(v*) in the system is estimated to be similar to 1.4% in CH4. According to theoretical calculations reported previously, the rate coefficient for the reaction Cl+CH4(v*) is much greater than that for Cl+CH4 at 298 K, especially at low temperatures (10-235 times at 200 K); formation of CH4(v*) in the Cl+CH4 system can thus explain why rate coefficients determined previously through flash photolysis near 220 K are similar to 20% greater than those determined in a discharge-flow system. (C) 2001 American Institute of Physics.