Knowledge Bank

The CaX molecules are the alkali atoms of diatomic molecular spectroscopy. The electronic transitions of these ionic radicals occur between singly-occupied, non-bonding metal-centred molecular orbitals. The molecular parameters are direct probes of the orbital character. A simple quantitative semi-empirical model will be described. This model provides an explanation for almost all of the known spectroscopic constants of the X, A and B states, including A-doubling, spin-rotation, spin-orbit, and lifetime and hyperfine constants. The hydrogenic model indicates that the $X2\Sigma +$ state is about 75% 4s$\sigma (Ca+$) and 25 % 4p$\sigma (Ca+)$ while the $B2\Sigma +$ state is about 60% 4p$\sigma (Ca+)$ and 40% 3d$\sigma (Ca+)$ for the entire CaX series. The $A2\Pi$ state is almost pure 4p$\pi$ in CaF (91%), but becomes increasingly mixed with 3d$\pi$ along the CaX series until for CaI the A state is only 70% 4p$\pi$.