Characterisation of the ground XΠ and first excited AΣ electronic states of MgO by high-resolution photoelectron spectroscopy.

Despite the importance of MgO for understanding the electronic structure and chemical bonds in alkaline-earth metal oxides and its potential astrophysical relevance, hardly any spectroscopic information is available on this molecular cation. We report on a high-resolution photoelectron spectroscopic study of MgO using a resonant (1 + 1') two-photon excitation scheme in combination with PFI-ZEKE photoelectron spectroscopy. By carrying out the resonant excitation selected rotational levels of several intermediate states of different electronic configurations, total electronic spins, and internuclear distances, a broad range of vibrational levels of the XΠ ( = 3/2, 1/2) ground and AΣ first excited states of MgO were observed for the first time. The new data provide a full characterisation of the rovibronic level structure of MgO up to 2 eV (16 000 cm) of internal energy. A full set of vibrational, rotational and spin-orbit-coupling molecular constants were extracted for these two electronic states. The adiabatic ionisation energy and the singlet-triplet interval of MgO were determined to be 64 577.65(20) cm and 2492.4(3) cm, respectively.