The York Research Database

A single band belonging to the à Δ- X̃ band system has been rotationally analyzed for each of the two isotopologues, BaOH and BaOD, using high-resolution V-type optical-optical double resonance spectroscopy. BaOH and BaOD molecules were synthesized in a Broida-type oven. High-resolution spectra were recorded by monitoring the dip in fluorescence of the B - X transition excited by a single-mode ring dye laser (pump laser), whilst a single-mode Ti:Sapphire laser scanned the corresponding à Δ-X̃ transition. The observed spectra resemble a typical Π- Σ transition, believed to emanate from single or triple quanta of the bending vibration in the Ã Δ 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 state. Previous analyses of the à Π-X̃ 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 Δ state. Although the newly observed A Δ state bands have not been conclusively assigned a specific spin state, the derived Ω-doubling constants show significant Π1 character, further indicating strong interactions between the A Π and A Δ 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 D̃ , D̃ , C̃ Π, B̃ , à Π, Ã Δ and X̃ states have been characterized by means of multireference configuration interaction calculations using the MOLPRO software. Calculated vertical term energies show relatively good agreement with existing optical data.