Knowledge Bank

The CaO Green and Orange Systems are so complex that their attribution to CaO rather than a polyatomic species was in doubt until recently. Excluding all $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma +$ states, the electronic structure of CaO may be understood as $Ca+O-$ where the $Ca+$ structure $(X0,A\pi ,B\sigma Ca+$ centered orbitals) is well represented by a ligand field model for the $Ca-monohalides1$ and an O- p-hole ($\sigma -1$ or $\pi -1$). All prominent features of the CaO Orange system have been assigned as transitions between five $A-1$ upper states and two $X\sigma \mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ lower states $(a3\pi ,A\prime 1\pi )$. The model predicts that the Green System will consist of $B0\pi \mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}\mathrm{<mrow\; data-mjx-texclass="ORD">1,3</mrow>}\pi i-x\sigma \pi \mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}\mathrm{<mrow\; data-mjx-texclass="ORD">1,3</mrow>}\pi i$ transitions analogous to the Green CaF $B2\Sigma +-X2\Sigma +$ system. Laser excitation-resolved fluorescence spectroscopy has confirmed the predicted assignments of the upper and lower states of the CaO Green System. Rotation-vibration analysis of this exceedingly congested system requires a sub-Doppler Optical Optical Double Resonance (OODR) scheme linked via a common lower level in order to select a single J, e/f, component. Preliminary measurements show $TO$ and $BO$ values for the $B\sigma \pi -1$ state of 26 $348cm-1$ and $0.353(4)cm-1$. This newly discovered 1* state lies only $435cm-1$ above and has a $BO$-value $0.033cm-1$ (i.e. 9%) smaller than the known $B1\pi$ state. This is in accord with the $A\pi \sigma \mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ configuration of $B1\pi$ and provides further evidence for the almost perfect separation of the CaO electronic spectrum into $\pi -1$ and $\sigma -1$ sub-manifolds.