Analysis of VUV and optical spectra of Cs2NaYF6 crystals doped with Tm

This study presents a detailed interpretation and analysis of the reported interconfigurational spectra of Tm³⁺ in Cs₂NaYF₆ [V.N. Makhov, N.M. Khaidukov, D. Lo, J.C. Krupa, M. Kirm, E. Negodin, Opt. Mater. 27 (2005) 1131]. Since only spin-forbidden d–f emission bands are observed for Tm³⁺ in this host, it is unsuitable for use as a scintillator because the emission lifetime is too long. The d–f emission spectrum is well-explained by calculation and most of the intensity is located in one band: 4f¹¹5d¹ (high-spin) → 4f^{12 3}H₆. Measurements from the bands in the d–f excitation spectrum do not provide the accurate separation energy of the high and low-spin states. Strict O_h point group selection rules are operative for the optical spectra. The electronic states of the 4f¹¹5d¹ configuration (briefly, d-electron states, hereafter) are calculated to span from 58,318 cm⁻¹ to 86,900 cm⁻¹. At least five structured bands are observed in the excitation spectrum and their intensities are fairly well simulated by calculation. The 4f¹² → 4f¹³ (np₆)⁻¹ charge transfer band is assigned between 140 and 120 nm (83,000–71,000 cm⁻¹) and excitation into this band leads to f–f emission, bypassing the d-electron states.