Knowledge Bank

We have observed the $\nu 3$ fundamental vibration-rotation band of $C2H2+(2\Pi \ast )$. This band was among the very rich hydrocarbon spectra in the $3.3\mu m$ region which also yielded the spectra of $CH3+1$ and $C2H3+2$. It was observed using a difference frequency spectrometer and velocity modulation for detection. This ion was produced in an AC glow discharge with a gas mixture of He:$H2:C2H2(700:20:1)$ with a total pressure of $\approx 6Torr$. The spectral lines of $C2H2+$ could be discriminated from those of $CH3+$ and $C2H3+$ by their different chemical behavior. We have determined the preliminary molecular constants $as3$ (in $cm-1$):[FIGURE]