Fluorescence lifetimes of the ùΠ_u state of C₃

The fluorescence lifetimes of 115 vibrational levels of the ùΠ_u state of C₃ have been measured under supersonic molecular beam conditions. Of these, ninety-one are Π_u vibronic levels, for which the lifetimes lie in the range 190-700 ns. The lifetimes of those Π_u levels where only the bending vibration is excited lie in the range 190-235 ns. There is very little variation with bending quantum number, and the lifetimes of the two orbital components of the ¹Π_u state are essentially the same. When ν₁ and ν₃ are excited, the lifetimes become longer and/or reach a maximum for levels with v₁ + v₃ ∼ 4. Excitation of the bending vibration in addition to the stretching vibrations shortens the lifetime slightly. Several of the levels show double-exponential decays. Another 23 levels, of Σu⁺ vibronic symmetry, mostly have lifetimes that are longer than 300 ns. Interaction with nearby "dark" electronic states, such as B̃¹Σ u^-, B̃′¹Δ_u, C̃¹Π_g, and b̃³Π_g, is proposed to account for the observed lifetime lengthening. A particularly clear instance of such an interaction is the long lifetime (914 ns) of a perturbing Σu⁺ level at 30 181 cm^-1, which is confirmed as belonging to the perturbing B̃′¹Δ_u state. A single level of Δ_u symmetry at 29 170 cm^-1, which perturbs one of the Π_u levels, is shown to belong to the à state.