ORANGE BANDS OF CaO: OODR SPECTROSCOPY AND ELECTRONIC STRUCTURE MODEL

$^{1}$ R.F.Marks, R.A. Gottscho, and R. W. Field, Physica Scripta 25, 312 (1982). $^{2}$ B. Norman, K.Cross. H.Schweda, M. Polak, and R. W. Field; unpublished.Author Institution: Department of Chemistry, Massachusetts Institute of TechnologyAlthough the electronic spectrum of CaO is extremely complex and congested ($>10$*** lines within $700 cm^{-1}$ in the Orange Bands), its electronic structure is so simple that it is likely to serve as a zero-order, atomic-ion-in-molecule model for the electronic structure of the transition metal monoxides. With the exception of the nominally $Ca^{2}O^{2}- X^{1}\Sigma^{\prime}$ state, the electronic structure of CaO may be understood as a weakly interacting electro/hole pair, the electron localized on $Ca^{+}$ (in $X\sigma, A\pi B\sigma$ orbitals borrowed from the corresponding $X^{2}\Sigma, A^{2}\Sigma, B^{2}\Sigma^{+}$ states of CaF) and the hole localized on $O 2p^{-1}(\sigma^{-1}$ or $\pi^{-1}$ as for NaO). We report here the previously unknown $C***\Sigma$ state, belonging to the $A\pi \pi^{-1}$ configuration. This completes the characterization of the CaO Orange Bands which consist of transitions between all six $A\pi \pi^{-1}(^{1,3}\Delta {^{1,3}}\Sigma^{+})$ upper states and the two $X\sigma \pi^{-1}(A^{1}\Pi, a{^{3}}\Pi)$ lower $states.^{1,2}$ Experimental knowledge of the four lowest $^{1}\Sigma^{\prime}$ states $(X^{1}\Sigma^{+}-Ca^{2}O^{2}-, A^{1}\Sigma - X\sigma \sigma^{-1}, C^{1}\Sigma - A\pi \pi^{-1}, C^{1}\Sigma^{+} - B\sigma \sigma^{-1})$ should provide stringent tests of ab initio calculations as well as an opportunity to extract deperturbed, integer-valence, $Ca^{2+} O^{2-}$ and $Ca^{+}O^{2-}O$ $^{1}\Sigma^{+}$ potential curves, energy separations, and intervalence interaction strengths. A lower level linked OODR scheme has allowed unambiguous J, e/f, and electronic state labelling of the $C^{-1}\Sigma \leftarrow A^{-1}\Pi$ band system in the CaO Orange Bands. This same technique was used to obtain sub-Doppler spectra from which accurate constants for both upper and lower states have been derived. The $^{1}\Sigma$ identity of the upper state in this transition was confirmed by observation of resolved fluorescence spectra in the $C^{-1}\Sigma \rightarrow X^{1}\Sigma^{+}$ transition. The preliminary molecular constants for the $C^{-1}\Sigma \rightarrow X^{1}\Sigma^{+}$ state $T, B, \alpha$, and $\omega$, are $24 635.546, 0.3456, 2.4 \times 10^{5}$, and 549.64 respectively (all in $cm^{-1}$)