Inverse bottleneck in Eu2+–Mn2+ energy transfer

The energy transfer (ET) between Eu2+ and Mn2+ in Ca5(PO4)3Cl has been investigated. At low intensities under 405 nm excitation, time-resolved experiments provide microscopic parameters for the energy transfer between adjacent Eu2+ and Mn2+ ions. At high intensities, we observe a non-linear component in the energy transfer process due to ground state depletion of the Mn2+ ions, leading not to a reduction, but to an increase in the energy transfer rate between Eu2+ and Mn2+ ions and also energy transfer between excited Mn2+ ions. This results in a sub-linear response of both Eu2+ and Mn2+ luminescence as a function of intensity. Our observations are quantitatively described by a model using energy transfer to Mn2+ ions in both the ground and the excited state.