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