Rate coefficients for reaction and for rotational energy transfer in collisions between CN in selected rotational levels (XΣ+2, v=2, N=, 1, 6, 10, 15, and 20) and C2H2

Abstract

Rate coefficients (ktot,Ni) are reported (a) for total removal (reactive+inelastic) of CN(X2Sigma+,v=2,Ni) radicals from selected rotational levels (Ni=0, 1, 6, 10, 15, and 20) and (b) for state-to-state rotational energy transfer (ki-->f) between levels Ni and other rotational levels Nf in collisions with C2H2. CN radicals were generated by pulsed laser photolysis of NCNO at 573 nm. A fraction of the radicals was then promoted to a selected rotational level in v=2 using a tunable infrared "pump" laser operating at approximately 2.45 microm, and the subsequent fate of this subset of radicals was monitored using pulsed laser-induced fluorescence (PLIF). Values of ktot,Ni were determined by observing the decay of the PLIF signals as the delay between pump and probe laser pulses was systematically varied. In a second series of experiments, double resonance spectra were recorded at a short delay between the pump and probe laser pulses. Analysis of these spectra yielded state-to-state rate coefficients for rotational energy transfer, ki-->f. The difference between the sum of these rate coefficients, Sigmafki-->f, and the value of ktot,Ni for the same level Ni is attributed to the occurrence of chemical reaction, yielding values of the rotationally selected rate coefficients (kreac,Ni) for reaction of CN from specified rotational levels. These rate coefficients decrease from (7.9+/-2.2)x10(-10) cm3molecule-1 s-1 for Ni=0 to (0.8+/-1.3)x10(-10) cm3 molecule-1 s-1 for Ni=20. The results are briefly discussed in the context of microcanonical transition state theory and the statistical adiabatic channel model.