Lifetimes of the ∼{A} States of C_3

Fluorescence lifetimes have been measured for 66 Π_u and 16 Σ^+_u vibronic levels of the ∼{A}^1Π_u state of C_3 under nearly collision-free conditions. The vibrational levels studied were v_1=0-5, v_2=0-22, and v_3=0-4, covering the energy range up to 6000 cm^{-1}, and for most of them only the R(0) line was investigated. The measurements are estimated to be accurate to better than ±3 ns. General features of the lifetimes are as follows: Pure bending levels with Π_u-symmetry have shorter lifetimes (as short as 190 ns). With increasing excitation of the stretching vibrations, the lifetimes increase. Bending excitation generally reduces the lifetimes, though exceptions occur for v_1=1. Levels with v_1+v_3 ≥ 3 or with vibrational energy greater than 4000 cm^{-1} have lifetimes longer than 300 ns; some of these levels even have a long-lived component with a lifetime of up to several microseconds. This implies that the predicted dark states, (∼{B}^1Σ_u^- and ∼{B}' ^1Δ_u), lie at least 4000 cm^{-1} above the 0 0 0 level of the ∼{A} state. It is interesting to note that most of the Σ_u^+-symmetry levels lying above 4000 cm^{-1} also have lifetimes longer than 300 ns.