Optical spectra and excited state relaxation dynamics of Sm²⁺ ions in SrCl₂, SrBr₂ and SrI₂ crystals

Samarium(2+) doped single crystals of SrCl₂, SrBr₂ and SrI₂ with a nominal samarium concentration of 0.12, 0.16 and 0.13 at%, respectively, were grown by the Bridgman method. XRD examination revealed that obtained samples are single phase and their structures are consistent with those intentional. Based on preliminary spectroscopic measurement it was ascertained that Sm²⁺ ions are incorporated without traces of Sm³⁺ contamination. High resolution absorption, luminescence and excitation spectra as well as luminescence decay curves were measured as a function of temperature in the 4.2–300 K region. It was found that structural peculiarities of the host crystals affect strongly spectroscopic features of incorporated Sm²⁺ ions. At room temperature all systems under study show a broad-band luminescence related to the parity allowed inter-configurational 4f⁵5d¹ – 4f⁶ transitions of Sm²⁺. At 5 K a broad band luminescence vanishes in SrCl₂:Sm²⁺ and a resulting luminescence spectrum shows a rich fine structure consisting of narrow lines that we attribute to magnetic dipole transitions within the 4f⁶ configuration of Sm²⁺ ions located in centro-symmetric sites and/or their vibrational side bands. Based on an analysis of luminescence decay curves we attribute numerous narrow lines that appear in luminescence spectra of SrBr₂:Sm²⁺ recorded below 25 K to the ⁵D₀ – ⁷F_J (J = 0–4) intra-configurational transitions of Sm²⁺ ions located in sites with C₄ and C₁ symmetry. A long-lived luminescence related to intra-configurational transitions was not observed in SrI₂:Sm²⁺ even at 4.2 K.