LIFETIMES OF THE $\tilde{A}$ STATES OF C$_3$

Author Institution: Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei 10617, Taiwan, R. O. C.Fluorescence lifetimes have been measured for 66 $\Pi_u$ and 16 $\Sigma^+_u$ vibronic levels of the $\tilde{A}^1\Pi_u$ 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 $\pm$3 ns. General features of the lifetimes are as follows: Pure bending levels with $\Pi_u$-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$_3$ $\geq$ 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, ($\tilde{B}^1\Sigma_u^-$ and $\tilde{B}^\prime$ $^1\Delta_u$), lie at least 4000 cm$^{-1}$ above the 0 0 0 level of the $\tilde{A}$ state. It is interesting to note that most of the $\Sigma_u^+$-symmetry levels lying above 4000 cm$^{-1}$ also have lifetimes longer than 300 ns