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 (2)Σ(+) ground state via the E (2)Π(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-î † photoelectron partial waves in the photoionisation of the E (2)Π(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.