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

The temperature dependence of the reactions of the dicarbon molecule in its ground singlet (X (1)Sigma(+)(g)) and first excited triplet (a (3)Pi(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 <= T <= 220 K and measurements have been performed at 296 K under subsonic flow conditions. C-2 was produced in its two lowest electronic states via the in situ multiphoton dissociation of C2Br4 at 266 nm. The time dependent losses of C-2 in these two states in the presence of an excess of coreagent species were simultaneously followed by laser-induced fluorescence in the Mulliken and Swan bands for the detection of singlet and triplet state C-2, respectively. The rate coefficients were measured to be very fast, with values larger than 10(-10) cm(3) molecule(-1) s(-1) and up to 5 x 10(-10) cm(3) molecule(-1) s(-1). The reactions of C-1(2) are seen to be essentially temperature independent from 77 <= T <= 296 K whereas the rate coefficients for the C-3(2) reactions are seen to increase until they are equivalent to the C-1(2) values at 77 K.