Knowledge Bank

Since the discovery of the $B2\Sigma u+-X2\Sigma g+$ electronic transitions of $C2-$ in the optical region by Herzberg and $Lagerqvist1$, many experimental and theoretical works have been reported for this interesting molecular ion. While both the $X2\Sigma g+$ and $B2\Sigma u+$ states have been well understood, the only information on the $A2\Pi u$ state is from the optical photo-detachment cross-section study by $Mead2$ et al., in which they obtained the information from the pertubations on the $B2\Sigma u+$ energy levels. We observed the high resolution absorption spectrum of the $C2-A2\Pi u-X2\Sigma g+$ band as part of our project to study carbo-ions. The spectrum was observed in an ac discharge of $C2H2$ and He using the velocity modulation method. Strong lines were observed with a signal to noise ratio of up to 100. About 50 lines have been measured and assigned. Our preliminary molecular constants of the $A2\Pi u$ states, together with the results by Mead et al., are listed below: [FIGURE] This work Previous work $v003930.288(42)4002(180)B1.6349(5)1.622(5)D6.33(87)\times 10-6-A-24.977(84)-24(1)8.6(51)\backslash time10-4$ - We have also observed the (1,1) hot band with good signal to noise ratio indicating very high vibration temperature of $C2-$ in the discharge. The analysis of this band will be presented.