AΣ1→Σ1Transition of theC2Molecule

The $\lambda 2300$ "band" of the carbon arc, previously noticed by Bloomenthal and other investigators and measured by Hori, who attributed it to a ${\mathrm{C}}_3$ molecule, has been accurately measured and analyzed, using photographs taken on the 30-foot, 30,000-line grating spectrograph. The analysis shows definitely that the structure consists of superposed (0,0), (1,1), (2,2), and (3,3) headless bands of a $\Sigma \to \Sigma$ transition of ${\mathrm{C}}_2$ (or possibly $\mathrm{C}_2^{}{}_{}{}^{+}$), in agreement with the earlier diagnosis of Mulliken and Dieke. Alternate lines are missing in each series, as expected in view of the zero spin of the carbon nucleus. Most probably the transition is ${}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{+}\to {}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{+}$ of ${\mathrm{C}}_2$. The molecular constants are very nearly equal for the upper and lower electronic states. They correspond rather closely to the average values of the same constants for other known states of ${\mathrm{C}}_2$. The following values were obtained for the more important constants: $B_e^{}{}_{}{}^{\prime }=1.8334\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B_e^{}{}_{}{}^{\prime \prime }=1.8223\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\alpha }^{\prime }=0.0204\mathrm{}$, ${\alpha }^{\prime \prime }=0.0195\mathrm{}$, $r_e^{}{}_{}{}^{\prime }=1.2382\mathrm{}$ A, $r_e^{}{}_{}{}^{\prime \prime }=1.2419\mathrm{}$ A, $\omega _e^{}{}_{}{}^{\prime }=1748\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\omega _e^{}{}_{}{}^{\prime \prime }=1774\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\nu _0^{}{}_{}{}^{0,0}=43,227.25\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The ${\omega }_e$ values were obtained indirectly from the $B$ and