Investigating the electronic states of BaOH by V-type double resonance spectroscopy and ab initio calculations: Further evidence of perturbation from the A∼′2Δ state

► A∼′2Δ-X∼2Σ+ transitions of BaOH/BaOD are analyzed by double resonance spectroscopy. ► Two bands are tentatively assigned to the A∼′2Δ3/2(010)-X∼2Σ+(000) transitions. ►Measured lines are assigned and rotational and fine structure parameters determined. ► Results indicate further interactions between the A∼2Π and A∼′2Δ states of BaOH. ► Calculated term energies of low-lying states show good agreement with optical data.