The luminescence and scintillation properties of SrI ₂ single crystals doped with 5% Eu ²⁺ and 0.05%, 0.2% and 0.5% Sm ²⁺ are evaluated. X-ray excited and photoluminescence measurements show energy transfer from excited Eu ²⁺ ions to Sm ²⁺ ions. At a concentration of 0.5% Sm ²⁺ , the luminescence consists almost entirely of 740 nm emission from Sm ²⁺ 5d-4f transitions. Co-doping SrI ₂ :5% Eu ²⁺ with Sm ²⁺ provides a novel method to bypass the self-absorption problem encountered in large SrI ₂ :Eu ²⁺ crystals and, at the same time, provides a unique near-infrared emitting scintillator with a light yield of approximately 40,000 photons/MeV.

This paper provides an overview and interpretation of the spectroscopic data of the Bi³⁺ activator ion in 117 different inorganic compounds. The energies of the metal-to-metal charge transfer and the interconfigurational transitions of Bi³⁺ were collected from the archival literature. Using these energies, in combination with the electron binding energies in the host conduction and valence band, the binding energies in the 6s ground state and 6p excited state were determined relative to the vacuum level. The locations of the Bi³⁺ energy levels within the forbidden gap of the host compound provides valuable insight in the physical properties of the Bi³⁺ activator ion in different compounds.

An overview of the spectroscopic data on Tl⁺ and Pb²⁺ in 37 and 126 different inorganic compounds, respectively, is presented. Using the metal-to-metal charge transfer and A-band transition energies, the electron binding energies in the S01 ground state and P13 excited state of Tl⁺ and Pb²⁺ were determined relative to the vacuum level. By constructing vacuum referred binding energy (VRBE) diagrams that display the energy levels of the Tl⁺ and Pb²⁺ activator ion together with that of the host compound, insight in the luminescent properties of these activated compounds are gained. The obtained VRBEs of the electron in the S01 and P13 states of Tl⁺ and Pb²⁺ are compared to those of Bi³⁺.

The vacuum referred binding energy of the electron in the Bi²⁺ ground state has been determined in 15 different compounds. This shows that the electron binding energy in the ground state of Bi²⁺ is at lower (more negative) energy compared to the electron binding energy in the excited state of Bi³⁺. This means that electron transfer from the excited state of one Bi³⁺ to a neighboring Bi³⁺, forming a Bi2+−Bi⁴⁺ pair, acts as a quenching route for the Bi³⁺ emission. Electron back transfer in the Bi2+−Bi⁴⁺ pair is then suggested to be the origin for the frequently observed pair emission. This paper shows that vacuum referred electron binding energy diagrams can provide a unique physical insight in the properties of inorganic compounds.

Bismuth-doped Li₂BaP₂O₇ was prepared in air, showing no Bi³⁺ or Bi²⁺ related photoluminescence even at 10 K. Absorption measurements showed that only Bi³⁺ was present in the as-prepared samples of which the emission is completely quenched. During X-ray excitation the characteristic deep-red radioluminescence of Bi²⁺ was observed. After X-ray irradiation, this red luminescence of Bi²⁺ could be excited optically, indicating that upon X-ray irradiation the Bi³⁺ is reduced to Bi²⁺. Based on the spectroscopic results, the Bi³⁺ and Bi²⁺ energy levels were estimated in a vacuum referred binding energy (VRBE) scheme and were used to explain the observed luminescence behavior. The VRBE scheme provided an interpretation for the commonly observed Bi³⁺ pair emission in bismuth-doped compounds. In the case of Li₂BaP₂O₇:Bi it was used to explain the self-quenching behavior of Bi³⁺. These findings show that is possible to initially dope compounds with Bi³⁺ ions while only radioluminescence is observed from Bi²⁺ when the sample is exposed to high-energy excitation. This phenomenon can be used to fabricate new types of luminescent materials.