On the Cameron Bands (Π3−Σ1) of Carbon Monoxide

The 0-0 and 1-0 Cameron bands of CO are investigated in absorption with high dispersion. Three $P$, three $Q$ and three $R$ branches are observed for each band. New accurate values for the rotational constants of the ground state of CO are derived: $B_e=1.9310\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\alpha =0.01744\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and, using Bearden's values for $e$, $h$ and $N$, we obtain the moment of inertia $I_e=14.456\mathrm{}·{10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$, $I_0=14.521\mathrm{}·{10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$, the nuclear distance $r_e=1.127\mathrm{}$ A, $r_0=1.129\mathrm{}$ A. From the way in which the $\Lambda$ doublets of the $a{}_{}{}^3{}_{}{}^{}\Pi$ state combine with the ${}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ ground state it can be definitely shown that the upper state of the third positive group of CO (whose lower state is $a{}_{}{}^3{}_{}{}^{}\Pi$) is ${}_{}{}^3{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ not ${}_{}{}^3{}_{}{}^{}\Sigma _{}^{-}{}_{}{}^{}$. For the electronic excitation energy of the $a{}_{}{}^3{}_{}{}^{}\Pi$ level a value 48473.7 ${\mathrm{cm}}^{-1\mathrm{}}$ is obtained which differs to some extent from the values at present used in the literature.