Energy transfer in thermal and hyperthermal collisions between CN(X2Σ+, v = 2) in selected rotational levels (Ni = 0, 1, 6, 10, 15 and 20) and N2

Abstract

We report rate coefficients (k_tot,Ni) for total removal of CN(X²Σ⁺, v = 2, N_i) radicals from selected rotational levels (N_i = 0, 1, 6, 10, 15 and 20) and for state-to-state rotational energy transfer (k_i→f) between levels N_i and other rotational levels N_f in single collisions with N₂. CN radicals have been generated using two sources: (a) the pulsed laser photolysis of ICN at 266 nm, which generates translationally ‘hot’ CN radicals; and (b) the pulsed laser photolysis of NCNO at 570 nm, which generates CN radicals with translational energies close to the average value at 298 K. Comparison of the values of k_tot,Ni obtained using these two sources of CN demonstrates: firstly, that the same results are obtained as long as time is allowed for the translationally hot CN radicals generated from ICN to be thermalised before radicals are promoted to a specific rotational level in v = 2 using a tuneable infrared ‘pump’ laser operating at ca. 2.45 μm; and secondly, that the rate coefficients decrease, but the averaged cross-sections remain approximately constant, as the excess translational energy in CN radicals is moderated by collisions. With NCNO as the source of CN radicals, the observed values of k_tot,Ni do not depend on the delay between the pulses from the photolysis and pump lasers. Finally, we demonstrate that, for the non-reactive collision partner N₂ and with allowances made for the rate coefficients that are too small to measure directly, the sum of the state-to-state rate coefficients, Σ_fk_i→f, for rotational energy transfer from a selected initial level N_i agrees quite well with the value of k_tot,Ni for total transfer from the same initial level. The values of k_tot,Ni and of the state-to-state rate coefficients are compared with similar, earlier, results in which helium and argon were the collision partners. The relevance of these results to the study of collisions of CN with reactive collision partners is briefly discussed.