G. M. Salley, R. Valiente and H. U. Güdel
Luminescence upconversion mechanisms in Yb³⁺-Tb³⁺ systems
J. Lumin. 94-95, 305-309 (2001)                 

Abstract: Tb³⁺ (⁵D₄ ⁷F_J) luminescence has been observed in single crystals of SrCl₂: Tb³⁺ (1%): Yb³⁺ (1%) and Cs₃Tb₂Br₉: Yb³⁺ (1%) under excitation energies in the region of Yb³⁺ absorption. Previous reports of similar systems have postulated two possible upconversion mechanisms: (1) cooperative sensitization and (2) a sequence of two cooperative absorption steps, or GSA/ESA. We present results of emission, excitation and kinetic measurements at low and high temperatures. These measurements allow assignment of the mechanism responsible for upconversion in these systems. For temperatures greater than T = 100 K process (1) is the dominant mechanism, in agreement with previous assignments. However, for T < 100 K in Cs₃Tb₂Br₉: Yb³⁺ (1%), the upconversion occurs through a GSA/ESA sequence, which is shown here in a clear manner for the first time. The efficiency of the Yb³⁺-Tb³⁺ upconversion process for SrCl₂: Tb³⁺ (1%): Yb³⁺ (1%), at room temperature, under 2.4(10)⁴ W/cm², is on the order of 10^–4 and decreases with decreasing temperature by four orders of magnitude.