Absorption, light emission, and upconversion properties of Tm2+-doped CsCal3 and RbCal3

Absorption, light emission, and upconversion properties of Tm2+-doped CsCal(3) and RbCal(3) single crystals are presented and compared. Both compounds show multiple emissions after excitation at 21834 cm(-1) between 10 and 300 K. Besides sharp 4f - 4f transitions around 8800 cm(-1), five and three broad 4f - 5d emission bands are observed at higher energies in CsCal(3): Tm2+ and RbCal(3): Tm2+, respectively. The 4f - 5d transitions are very sensitive to the crystalline environment: the onset of the 4f - 5d excitations is red- shifted by about 1000 cm(-1) in RbCal(3): Tm2+ compared to CsCal(3): Tm2+. In addition, a broadening of bands is observed in the former compound. These differences are attributed to the structural changes that occur when the alkali metal is changed from Cs to Rb in these crystal lattices. An increased energy splitting of the multiplets and a red shift of the barycenter of the (4f) (12)( 5d)(1) electron configuration in RbCal(3): Tm2+ is the result. This affects not only the color of the visible emission, which turns from green in CsCal(3): Tm2+ to yellow in RbCal(3): Tm2+, but also the excited state dynamics. As a consequence, the dominant upconversion processes are different in the two compounds. Thus, the two title compounds nicely illustrate the influence of the structural environment on the optical spectroscopic properties of Tm2+.