Characterisation of the electronic ground states of BaH⁺ and BaD⁺ by high-resolution photoelectron spectroscopy

The rovibrational energy-level structures of BaH⁺ and BaD⁺ in their X^{+ 1}Σ⁺ electronic ground state have been characterised by pulsed-field-ionisation zero-kinetic-energy photoelectron spectroscopy following resonance-enhanced (1 + 1') two-photon excitation from the BaH/BaD X ²Σ⁺ ground state via the E ²Π_1/2 (v' = 0, 1) intermediate levels. A full set of rovibrational molecular constants for the BaH⁺ and BaD⁺ ground states has been derived for the first time and the adiabatic ionisation energies of BaH and BaD were determined to be 38 679.96(20) and 38 652.69(20) cm^-1, respectively. Photoelectron spectra recorded via E-state levels of selected rovibronic parity exhibit pronounced intensity alternations of transitions to rotational states of the cations with even- and odd-valued rotational-angular-momentum quantum number N⁺. This observation is interpreted by invoking dominant contributions of even- photoelectron partial waves in the photoionisation of the E ²Π_1/2 (v' = 0, 1) intermediate states of barium hydride. The lowest pure-rotational transition frequencies of BaH⁺ and BaD⁺ are derived from the photoelectron spectra which may help the detection of BaH⁺ in the microwave and millimetre-wave ranges.