Tunable KrF laser-induced fluorescence of C2 in a sooting flame

We have observed laser-induced fluorescence of two different band systems of C-2 in a flame, excited by a tunable KrF laser near 248 nm. The first comprises several P and R lines of the (1,0) band of the e (3)Pi(g) - a (3)Pi(u), Fox-Herzberg system, with fluorescence bands extending past 350 nm. The second is the band head region of the (7,1) band of the D (1)Sigma(u)(+) <-- B' (1)Sigma(g)(+) system, with fluorescence at 232 nm from D to the X (1)Sigma(g)(+) ground state. Neither band has been previously observed in any environment. The flame in these experiments is highly sooting, and the C-2 seen here is likely produced by laser vaporization of the soot with subsequent laser photolysis of a C-2 precursor. In a rich flame, this fluorescence could cause interferences in other studies such as KrF laser Raman scattering. Moreover, signal level calculations suggest native C-2 near 10 ppm could be readily observed using the Fox-Herzberg excitation. Raman measurements of major species (X greater than or equal to 0.01)in the same flame, using the KrF laser, are in good agreement with a model prediction.