Photoluminescence of Sm³⁺ Ions in MgS, CaS, SrS and BaS Phosphors

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Details of emission and excitation spectra of powder phosphors were obtained at various temperatures between 6 K and room temperature. The emission spectra consist of six groups of emission lines located at about 565, 605, 650, 710, 790 and 900 nm at 300 K. The position of each emission line shifts slightly towards shorter wavelengths in the order, MgS:Sm³⁺, CaS:Sm³⁺, SrS:Sm³⁺ and BaS:Sm³⁺. The first five groups are assigned to the transitions from the ⁴G_5⁄2 state to the ⁶H_J(J=5⁄2, 7/2, 9/2, 11/2, 13/2) states and the last group to the ⁶F_J(J=1⁄2, 3/2, 5/2) states in a Sm³⁺ ion. The excitation processes are explained as being due to three transitions; (i) the transitions within the 4f⁵ configuration in a Sm³⁺ ion, (ii) the electronic 4f⁵→4f⁴·5d transition in a Sm³⁺ ion and (iii) the fundamental absorption in host crystal, followed by the transfer of the excitation energy to an unexcited Sm³⁺ ion.