Time-resolved kinetic studies on quenching of NCO (A~2 SIGMA~+) by alkanes and substituted methane molecules

摘要

NCO radicals were produced by laser photolysis of CHBr_3 at 266 nm followed by the reaction of CH with N_2O. The radicals were then electronically excited from the ground electronic state to the A~2 SIGMA~+ state with a Nd:YAG laser pumped dye laser at 438.6 nm in the Q subband of A~2 SIGMA~+ (00~0 0)<-X~2PI_i (00~10). The rate constants k_q and thermally averaged cross sections sigma_q for collision quenching of NCO (A~2 SIGMA~+) by n-C_nH_(2n+2) (n=1,5-8), c-C_6H_(12), CH_(4-n)Cl_n (n=1-4), CH_3OH, CH_3NO_2, and CCl_2F_2 were measured at room temperature (298 K) by observing the time-resolved fluorescence signals of the excited NCO in a cell at total pressure of about 20 Torr. Formation cross sections of complexes of the electronically excited NCO radicals and quenchers were calculated by means of a collision complex model. It was shown that the quenching rates of NCO (A~2 SIGMA~+) by alkane molecules increase with the number of C-H bonds of the molecules, and that the attractive forces play a main role in the collisional quenching processes of NCO (A~2 SIGMA~+) by the quenchers studied in this work.