Knowledge Bank

Optical optical double resonance (OODR) spectroscopy is a valuable tool, circumventing one photon electric dipole selection rules, in accessing highly excited as well as low lying metastable electronic states. In BaO both main and extra lines of an $A1\Sigma +$ rotational level perturbed by $a3\Pi$ are excited. From these mixed energy levels, access to either singlet states or triplet states is achieved by using a second tunable dye laser; fluorescence is then observed back down to $ a^{3}\Pi, A^{\prime},^{1}\Pi $ and $A^{\prime}\Sigma^{+} $. Double resonances are detected in a variety of ways: by monitoring ultraviolet fluorescence; by monitoring single resonance fluorescence from the pulped $A1\Sigma +$ level; and by monitoring red fluorescence dependent on both lasers. These experiments further characterize the metastable $a3\pi$ and $A\prime 1\Pi$ states. It also appears that BaO undergoes a dramatic change from Hund’s case ‘a’ to case ‘c’ coupling at excitation energies between 2 and 4 eV.