Ionization energy measurements and spectroscopy of HfO and HfO+

Rotationally resolved spectra for the HfO+ cation have been recorded using the pulsed field ionization zero electron kinetic energy (PFI-ZEKE) technique. Resonant excitation of the F(0(+))<- X (1)Sigma(+) band system of HfO was used as an intermediate level providing molecule and rovibrational state selectivity in the ionization process. The ionization energy (IE) of HfO, derived from the PFI-ZEKE spectrum, was determined to be 7.916 87(10) eV, which is 0.37 eV higher than the value reported from electron impact measurements. Underestimation of the IE in the previous studies is attributed to ionization of thermally excited states. A progression in the HfO+ stretch vibration up to nu(+)=4 was observed in the PFI-ZEKE spectrum, allowing for the determination of the ground electronic state vibrational frequency of omega(+)(e)=1017.7(10) cm(-1) and anharmonicity of omega(e)x(e)(+)=3.2(2) cm(-1). The rotational constant of HfO+ was determined to be 0.403(5) cm(-1). Benchmark theoretical ab initio calculations were carried out in order to explore the effects of electron correlation on the predicted molecular properties. Survey scans utilizing laser induced fluorescence and resonance enhanced multiphoton ionization detection revealed many previously unassigned bands in the region of the F-X and G-X bands of HfO, which we attribute to nominally forbidden singlet-triplet transitions of HfO.