Oliver S. Wenger, Claudia Wickleder, Karl W. Krämer and Hans U. Güdel
Upconversion in a divalent rare earth ion: optical absorption and luminescence spectroscopy of Tm²⁺ doped SrCl₂
J. Lumin. 94-95, 101-105 (2001)                 

Abstract: Crystals of 2% Tm²⁺ doped SrCl₂ were studied by variable temperature optical absorption and luminescence spectroscopy. The absorption spectrum consists of the weak 4f¹³ intraconfigurational ²F_7/2 -> ²F_5/2 transition around 8840 cm^–1 and a series of intense parity allowed f d transitions above 15000 cm^–1. Luminescence is observed from the 4f^{13 2}F_5/2 excited state as well as from the lowest-energetic 4f¹²5d excited state at all temperatures between 10 and 300 K. Upconversion luminescence from the latter is observed after cw- (lamp) excitation at 8840 cm^–1 into ²F_5/2 at 15 K, as well as after pulsed (laser) excitation at 1064 nm at 300 K.