Characterisation of the ground X$^+$ $^2\Pi_{\Omega}$ and first excited A$^+$ $^2\Sigma^+$ electronic states of MgO$^+$ by high-resolution photoelectron spectroscopy

Abstract

Despite the importance of MgO$^+$ for understanding the electronic structure and chemical bonds in the 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$^{\prime}$ 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 (16000 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 65474.7(2) cm$^{-1}$ and 2492.4(3) cm$^{-1}$, respectively.