Knowledge Bank

Two bands for the $A~\prime 2\Delta -X~2\Sigma +$ transitions of BaOH and BaOD have been rotationally analyzed using high-resolution V-type optical-optical double resonance spectroscopy. BaOH and BaOD molecules were synthesized in a Broida-type oven, using a single mode Ti:Sapphire laser and a single mode dye laser for molecular excitation. The observed spectra mimic a typical $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi -2\Sigma +$ transition, believed to emanate from single or triple quanta of the bending vibration in the $A~\prime 2\Delta$ state. Measured rotational lines have been assigned and rotational and fine structure parameters determined through a combined least-squares fit with the millimeter-wave pure rotational data of the $X~2\Sigma +$ state. Previous analyses of the $A~2\Pi -X~2\Sigma +$ transitions of BaOH and BaOD yielded significantly different spin-orbit coupling constants, which were attributed to possible global and local perturbations arising from vibrationally excited bands of the $A~\prime 2\Delta$ state. Although the newly observed $A~\prime 2\Delta$ state bands could not be conclusively designated a specific spin state, the derived $\Lambda$-doubling constants also show significant $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ character, further indicating a strong interaction between the $A~2\Pi$ and $A~\prime 2\Delta$ states of BaOH. To validate these conclusions, $ab\; initio$ calculations have been carried out to further understand the nature of the BaOH excited states. The wavefunctions of the $D~\prime 2\Sigma +$, $D~2\Sigma +$, $C~2\Pi$, $B~2\Sigma +$, $A~2\Pi$, $A~\prime 2\Delta$ and $X~2\Sigma +$ states have been optimised with a state averaged multiconfigurational calculation using the MolPro software. Calculated vertical term energies show relatively good agreement with existing optical data.