Infrared laser spectroscopy of jet-cooled carbon clusters: Structure of triplet C_6

We report the first structural characterization of the triplet isomer of C_6. Forty-one rovibrational/fine structure transitions in the v_4(σ_u) antisymmetric stretch fundamental of the C_6 cluster have been measured by diode laser absorption spectroscopy of a supersonic carbon cluster beam. The observed spectrum is characteristic of a centrosymmetric linear triplet state with cumulene-type bonding. The measured ground state rotational constant B_0 = 0·048 479 (10) cm^(-1) and the effective bond length r_(eff) = 1·2868 (1) Å are in good agreement with ab initio predictions for the linear triplet (^3Σ^-_g) state of C_6