Rate coefficients for reaction and for rotational energy transfer in collisions between CN in selected rotational levels (X2Σ+, v=2, N=0, 1, 6, 10, 15, and 20) and C2H2

Rate coefficients (k(tot,Ni)) are reported (a) for total removal (reactive+inelastic) of CN(X (2)Sigma(+),v=2,N-i) radicals from selected rotational levels (N-i=0, 1, 6, 10, 15, and 20) and (b) for state-to-state rotational energy transfer (k(i -> f)) between levels N-i and other rotational levels N-f 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 similar to 2.45 mu m, and the subsequent fate of this subset of radicals was monitored using pulsed laser-induced fluorescence (PLIF). Values of k(tot,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, k(i -> f). The difference between the sum of these rate coefficients, Sigma(f)k(i -> f), and the value of k(tot,Ni) for the same level N-i is attributed to the occurrence of chemical reaction, yielding values of the rotationally selected rate coefficients (k(reac,Ni)) for reaction of CN from specified rotational levels. These rate coefficients decrease from (7.9 +/- 2.2)x10(-10) cm(3)molecule(-1) s(-1) for N-i=0 to (0.8 +/- 1.3)x10(-10) cm(3) molecule(-1) s(-1) for N-i=20. The results are briefly discussed in the context of microcanonical transition state theory and the statistical adiabatic channel model. (c) 2007 American Institute of Physics.