Journal of Luminescence 94–95 (2001) 305–309 Luminescence upconversion mechanisms in Yb 3þ –Tb 3þ systems G.M. Salley a , R. Valiente b , H.U. Guedel a, * a Department f . ur Chemie and Biochemie, Universit . at Bern, Freistrasse 3, CH-3000 Bern 9, Switzerland b DCITIMAC, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain Abstract Tb 3þ ð 5 D 4 - 7 F J Þ luminescence has been observed in single crystals of SrCl 2 : Tb 3þ ð1%Þ : Yb 3þ ð1%Þ and Cs 3 Tb 2 Br 9 : Yb 3þ ð1%Þ under excitation energies in the region of Yb 3þ 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 ¼ 100 K process (1) is the dominant mechanism, in agreement with previous assignments. However, for T o100 K in Cs 3 Tb 2 Br 9 : Yb 3þ ð1%Þ; the upconversion occurs through a GSA/ESA sequence, which is shown here in a clear manner for the first time. The efficiency of the Yb 3þ –Tb 3þ upconversion process for SrCl 2 : Tb 3þ ð1%Þ : Yb 3þ ð1%Þ; at room temperature, under 2:4ð10Þ 4 W=cm 2 ; is on the order of 10 4 and decreases with decreasing temperature by four orders of magnitude. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Upconversion; Tb 3þ ; Yb 3þ 1. Introduction Green Tb 3þ emission under NIR Yb 3þ excita- tion was first observed by Livanova et al. in CaF 2 and SrF 2 doped with Yb 3þ and Tb 3þ [1]. The explanation of this process was based on the cooperative sensitisation of Tb 3þ by two Yb 3þ ions, i.e. 2 n Yb 3þ ð 2 F 5=2 Þ : Tb 3þ ð 7 F 6 Þ-2 n Yb 3þ - ð 2 F 7=2 Þ : Tb 3þ ð 5 D 4 Þ; shown schematically in Fig. 1(a) [2]. Near infrared (NIR) photons are absorbed by the Yb 3þ ions after which two excited Yb 3þ ions, which are near a Tb 3þ ion, non- radiatively transfer their energy to the Tb 3þ ion. Most authors, investigating the Yb 3þ –Tb 3þ sys- tem, have agreed with Livanova’s assignment [3–9]. However, at least two reports have proposed a process involving a ground state absorption (GSA) step followed by an excited state absorption (ESA) step within an Yb 3þ –Tb 3þ dimer [7,10]. Bilak et al. proposed that the upconver- sion (UC) process occurred by a sequence of two cooperative absorption steps [10], i.e. Yb 3þ ð 2 F 7=2 Þ : Tb 3þ ð 7 F 6 Þþ photon-Yb 3þ ð 2 F 5=2 Þ : Tb 3þ ð 7 F 6 Þþ photon-Yb 3þ ð 2 F 7=2 Þ : Tb 3þ ð 5 D 4 Þ: This assignment was based on UC kinetic measure- ments where the luminescence showed an immedi- ate decay after a short excitation pulse, which is a finger print of a GSA/ESA process [11]. In another report by Noginov et al., extra absorption lines in *Corresponding author. Tel.: +41-31-631-4249; fax: +41- 31-631-4399. E-mail address: hans-ulrich.guedel@iac.unibe.ch (H.U. Guedel). 0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII:S0022-2313(01)00310-6