The first experimental evidences are obtained by the mechanical spectroscopy, which yield forming the pair defects, i.e. primary nanoclusters of alloying element (AE)—excess vacancy (EV) at the early (pre-precipitate) stage in segregated solid solutions of as-cast eutectic alloys of Mg—Ba system. At the later stages, the clustering rate is increased by the presence of EV and completed by the stress rearrangement of AE—EV nanoclusters into magnesium clusters Mg$_n$ alloyed by the emission-active Ba$_m$ additions. It is noteworthy that thin structure of electron spectra is theoretically calculated for (Mg$_{16}$Ba$_2$) clusters using the first principles by the linearized augmented plane-wave (FLAPW) method with a complete approach for crystal-potential approximation of electronic structure of the x-ray photoelectron spectra (XPS) observed for massive (bulk) Mg—2% Ba (Mg$_{0,94}$Ba$_{0,06}$) alloy. Change of thin structure of XPS, occurrence of chemical shifts in Auger electron spectra (AES), and amplification of paramagnetic susceptibility of Mg—Ba alloys show that, as alloying element, Ba eliminates the minimum on a curve of electron density of states, $N(E)$, which is typical for the IIA subgroup semibrittle light metals, and raises the local density of engaged states near the Fermi surface ($E_{F}$). As revealed, embedded Mg$_{n}$Ba$_{m}$ clusters are responsible for hybridization of chemical bounds in segregated solid solutions with stabilizing the new hybrid $s$-, $p$-, $d$-electron configurations of clusters properly keeping their property to operate as emission-active centres raising a stable quantum yield of the photoemission. High quantum efficiency of the Mg—Ba alloys in a near UV-range of spectrum is explained by the surface-bulk character of the photoemission effect for eutectic alloys with clusterized structure having effective (at structural defects) mutual solubility of components (Mg and Ba).