Knowledge Bank

Fluorescence lifetimes have been measured for 66 $\Pi u$ and 16 $\Sigma u+$ vibronic levels of the $A~1\Pi u$ state of C$3$ under nearly collision-free conditions. The vibrational levels studied were v$1$=0$-$5,v$_2$=0$-$22,andv$_3$=0$-$4,coveringtheenergyrangeupto6000cm$^{-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$3$ $\ge$ 3 or with vibrational energy greater than 4000 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ 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, ($B~1\Sigma u-$ and $B~\prime$ $1\Delta u$), lie at least 4000 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ above the 0 0 0 level of the $A~$ state. It is interesting to note that most of the $\Sigma u+$-symmetry levels lying above 4000 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ also have lifetimes longer than 300 ns.