Probing electron transfer processes in YPO(4):Ce, Sm by combined synchrotron-laser excitation spectroscopy

Yttrium phosphate co-doped with cerium and samarium acts as a charge storage phosphor, but in highly doped material (0.5% co-doping levels), the proximity of defects leads to the uncontrolled non-radiative loss of stored charge through tunnelling. In order to characterize these defects, their mutual interactions and intra-pair charge transfer routes, experiments have been undertaken in which a laser probe is deployed during luminescence excitation using a synchrotron. Two modes of operation are described; in each case, the laser (2.8 eV) probes only Sm(2+) ions, and the detection is set to monitor exclusively Ce(3+) 5d-4f emission. Mode 1: the sample is pumped with monochromatic synchrotron photons in the range 4.5-12 eV, and the resultant charge populations probed with the laser 30 s later; this has the effect of sampling electrons trapped at Sm(2+) that are in quasi-equilibrium. Here, a clear transition between a sub-bandgap Urbach tail region and excitations above the mobility edge is especially apparent, enabling an accurate value of the conduction band energy of YPO(4) to be determined, 9.20 eV. Furthermore, the Sm(2+) and Ce(3+) ground state energies can be positioned within the bandgap (6.8 eV and 3.85 eV above the top of the valence band, respectively). Mode 2: the sample is pumped with monochromatic synchrotron photons in the range 4.5-12 eV and, during this pumping process, the laser probe is activated. This more dynamic process probes direct electron transfer excitation processes between spatially correlated Sm-Ce defect pairs, via their excited states; the laser probe enhances the Ce(3+) emission if direct electron transfer from the Ce(3+) ground state to the excited states of Sm(2+) is being pumped, or quenches the luminescence if the Ce(3+) excited states are pumped. The experiments allow for a precise measure of the difference in energy between the Sm(2+) and Ce(3+) ground states (2.98 eV).