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

Abstract

The rovibrational energy-level structures of BaH$^+$ and BaD$^+$ in their X$^+$~$^1\Sigma^+$ electronic ground state have been characterised by pulsed-field-ionisation zero-kinetic-energy photoelectron spectroscopy following resonance-enhanced ($1+1^\prime$) two-photon excitation from the BaH/BaD X~$^2\Sigma^+$ ground state via the E~$^2\Pi_{1/2}(v=0,1)$ intermediate levels. A full set of rovibrational molecular constants for the BaH$^+$and BaD$^+$ ground states have been derived for the first time and the adiabatic ionisation energies of BaH and BaD were determined to be 38679.96(20) and 38652.69(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-$\ell$ photoelectron partial waves in the photoionisation of the E~$^2\Pi_{1/2}(v=0,1)$ 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.