Characterisation of the ground X+ 2ΠΩ and first excited A+ 2Σ+ electronic states of MgO+ by high-resolution photoelectron spectroscopy

Abstract

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 ²⁴Mg¹⁶O were determined to be 64 577.65(20) cm⁻¹ and 2492.4(3) cm⁻¹, respectively.