An experimental study of the intersystem crossing and reactions of C2(X1Σg+) and C2(a3Πu) with O2 and NO at very low temperature (24-300 K)

A low-temperature gas-phase kinetics study of the reactions and collisional relaxation processes involving C-2(X-1 Sigma(+)(g)) and C-2(a(3)Pi(u)) in collision with O-2 and NO partners at temperatures from 300 to 24 K is reported. The experiments employed a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus to attain low temperatures. The C2 species were created using pulsed laser photolysis at 193 nm of mixtures containing C2Cl4 diluted in N-2, Ar, or He carrier gas. C-2(X-1 Sigma(+)(g)) molecules were detected via pulsed laser-induced fluorescence in the (D-1 Sigma(+)(u) <- X-1 Sigma(+)(g)) system, and C-2(a(3)Pi(u)) molecules were detected via pulsed laser-induced fluorescence in the (d(3)Pi(g) <- a(3)Pi(u)) system. Relaxation of C-3(2) by intersystem crossing induced by oxygen was measured at temperatures below 200 K, and it was found that this process remains very efficient in the temperature range 50-200 K. Reactivity of C-2(X-1 Sigma(+)(g)) with oxygen became very inefficient below room temperature. Using these two observations, it was found to be possible to obtain the C-2(X1 Sigma(+)(g)) state alone at low temperatures by addition of a suitable concentration of O-2 and then study its reactivity with NO without any interference coming from the possible relaxation of C-2(a(3)Pi(u)) to C-2(X-1 Sigma(+)(g)) induced by this reagent. The rate coefficient for reaction of C-2(X-1 Sigma(+)(g)) with NO was found to be essentially constant over the temperature range 36-300 K with an average value of (1.6 +/- 0.1) x 10(-10) cm(3) molecule(-1) s(-1). Reactivity of C-2(a(3)Pi(u)) with NO was found to possess a slight negative temperature dependence over the temperature range 50-300 K, which is in very good agreement with data obtained at higher temperatures.