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 XΣ 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 Σ ground state the E Π (' = 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, respectively. Photoelectron spectra recorded 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 . This observation is interpreted by invoking dominant contributions of even- photoelectron partial waves in the photoionisation of the E Π (' = 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.