Cross-relaxation quenching of x-ray excited luminescence in Eu-activated phosphors

Compounds, molecules, and nanoparticles containing lanthanides as primary constituents or as dopants are widely used in applications including luminescent dyes and lighting phosphors. Recent work has shown that x-ray spectroscopy methods can be used to monitor the sequence of excited states that leads to luminescence in lanthanide materials. Here, we use x-ray excited optical luminescence (XEOL) to identify a nonradiative process that quenches the emissive excited state of Eu${}^{3+}$ in the phosphors YVO${}_4$:Eu${}^{3+}$ and YVO${}_4$:Bi${}^{3+}$,Eu${}^{3+}$. Taking advantage of the high flux (up to $2 \times 10^{12}$ photons/second) and focusing capability (beam FWHM 5 $\mu$m) of a modern synchrotron beamline, we observe saturation of the XEOL yield at high x-ray flux densities. The saturation effect is interpreted with a kinetic model in which pairs of excited Eu ions undergo an Auger-like cross-relaxation. This effect is well documented in the literature on cathode-ray phosphors, and allows us to estimate the excited fraction of Eu${}^{3+}$ ions. We discuss applications of this method to the broader problem of studying energy transfer in luminescent materials, as well as technical implications for future x-ray spectroscopy studies that require high flux.