Characterisation of the ground X+2ΠΩ and first excited A+2Σ+ 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 via 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 X+ 2 Pi(Omega) (Omega = 3/2, 1/2) ground and A(+ 2)Sigma(+) 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(-1)) 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)-Mg-24-O-16 were determined to be 64 577.65(20) cm(-1) and 2492.4(3) cm(-1), respectively.