New photon upconversion processes in Yb3+ doped CsMnCl3 and RbMnCl3

Excitation around 980 nm and 935 nm leads to visible luminescence in the title compounds below 100 K. The broad luminescence band centered around 690 nm and 630 nm in Yb3+ doped CsMnCl3 and RbMnCl3, respectively, is identified as a 4T1→6A1 transition on Mn2+. The upconversion excitation spectrum follows the 2F7/2→2F5/2 absorptions around 980 nm and 935 nm. Excitation with 10 ns pulses induces instantaneous upconversion luminescence, indicating that no energy transfer is involved in the process. The most likely mechanism for this new type of upconversion process is based on exchange interactions between neighboring Yb3+ and Mn2+ ions. In a dimer picture the intermediate excitation is mainly, but not exclusively localized on Yb3+, whereas the upper emitting state is mainly but not exclusively localized on Mn2+.