Reaction kinetics to low temperatures. Dicarbon + acetylene, methylacetylene, allene and propene from 77 < or = T < or = 296 K.

The temperature dependence of the reactions of the dicarbon molecule in its ground singlet (X1Sigma(g)+) and first excited triplet (a 3Pi(u)) states with acetylene, methylacetylene, allene and propene has been studied using a recently constructed continuous supersonic flow reactor. Four Laval nozzles have been designed to access specified temperatures over the range of 77 < or = T < or = 220 K and measurements have been performed at 296 K under subsonic flow conditions. C2 was produced in its two lowest electronic states via the in situ multiphoton dissociation of C2Br4 at 266 nm. The time dependent losses of C2 in these two states in the presence of an excess of co-reagent species were simultaneously followed by laser-induced fluorescence in the Mulliken and Swan bands for the detection of singlet and triplet state C2, respectively. The rate coefficients were measured to be very fast, with values larger than 10(-10) cm3 molecule(-1) s(-1) and up to 5 x 10(-10) cm3 molecule(-1) s(-1). The reactions of 1C2 are seen to be essentially temperature independent from 77 < or = T < or = 296 K whereas the rate coefficients for the 3C2 reactions are seen to increase until they are equivalent to the 1C2 values at 77 K.