The2sΣ1→2pΣ1Bands of the Hydrogen Molecule

The so-called ${}_{}{}^1{}_{}{}^{}X\to 2p{}_{}{}^1{}_{}{}^{}\Sigma$ bands of ${\mathrm{H}}_2$ have been considerably extended and the analogous systems of HD and ${\mathrm{D}}_2$ discovered. The discovery of the new bands shows that the properties of the upper level of these bands are considerably different from those assumed until now, and the new facts allow a quite unambiguous identification of this level as $2s{}_{}{}^1{}_{}{}^{}\Sigma$. This state has an equilibrium distance of 1.0087·${10}^{-8\mathrm{}}$ cm slightly greater than the corresponding triplet level and is slightly less firmly bound. For the estimate of the significance and accuracy of the various constants the comparison of the three isotopic molecules is of great help. The $2s{}_{}{}^1{}_{}{}^{}\Sigma$ state shows marked class $B$ perturbations and the perturbing state is very probably ${\left(2p\sigma \right)}^2{}_{}{}^1{}_{}{}^{}\Sigma$ which should lie roughly 3500 ${\mathrm{cm}}^{-1\mathrm{}}$ above $2s{}_{}{}^1{}_{}{}^{}\Sigma$. This would explain also the fact that the $2s{}_{}{}^3{}_{}{}^{}\Sigma$ state does not show any perturbations.