Luminescence upconversion mechanisms in Yb3+-Tb3+ systems

Tb3+ (D-5(4) --> F-7(3)) 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)W-4/cm(2), is on the order of 10(-4) and decreases with decreasing temperature by four orders of magnitude. (C) 2001 Elsevier Science B.V. All rights reserved.