Correlation of Energy Levels of Linear and Bent Triatomic Molecules, and the Ultraviolet CS2Absorption Spectrum

A set of vibronic levels for a ${}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_g{}^{}$ and a ${}_{}{}^1{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ electronic state of linear A${\mathrm{B}}_2$ is shown in a diagram, together with gyrovibronic levels of the corresponding electronic states (a ${}_{}{}^1{}_{}{}^{}A_1^{}{}_{}{}^{}$ and a ${}_{}{}^1{}_{}{}^{}B_2^{}{}_{}{}^{}$ and a ${}_{}{}^1{}_{}{}^{}A_2^{}{}_{}{}^{}$) of bent A${\mathrm{B}}_2$; and a correlation is set up between the sets of gyrovibronic levels of the two cases. This diagram is then used in a discussion of the near-ultraviolet absorption bands of carbon disulfide, where the molecule is linear in the ${}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_g{}^{}$ ground state. The band types and structures for such a transition are discussed; the structures should be practically identical in character whether the upper state is linear or bent. Vibrational intensities, and their relation to a strong allowed electronic transition at shorter wave-lengths, are also discussed. It is concluded that the near-ultraviolet system is an electronic-allowed transition to a ${}_{}{}^1{}_{}{}^{}\mathrm{B}_2^{}{}_{}{}^{}$ bent molecule upper state related to a ${}_{}{}^1{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ linear molecule state.