Yttrium phosphate co-doped with cerium and samarium acts as a charge storagephosphor, but in highly doped material (0.5% co-doping levels), the proximityof defects leads to the uncontrolled non-radiative loss of stored charge throughtunnelling. In order to characterize these defects, their mutual interactions andintra-pair charge transfer routes, experiments have been undertaken in which alaser probe is deployed during luminescence excitation using a synchrotron. Twomodes of operation are described; in each case, the laser (2.8 eV) probes onlySm2 + ions, and the detection is set to monitor exclusivelyCe3 + 5d–4f emission. Mode 1: the sample is pumped with monochromatic synchrotronphotons in the range 4.5–12 eV, and the resultant charge populations probedwith the laser 30 s later; this has the effect of sampling electrons trapped atSm2 + that are in quasi-equilibrium. Here, a clear transition between a sub-bandgapUrbach tail region and excitations above the mobility edge is especiallyapparent, enabling an accurate value of the conduction band energy ofY PO4 to be determined, 9.20 eV.Furthermore, the Sm2 + and Ce3 + ground state energies can be positioned within the bandgap (6.8 eV and 3.85 eVabove the top of the valence band, respectively). Mode 2: the sample is pumpedwith monochromatic synchrotron photons in the range 4.5–12 eV and, duringthis pumping process, the laser probe is activated. This more dynamic processprobes direct electron transfer excitation processes between spatially correlatedSm–Ce defect pairs, via their excited states; the laser probe enhances theCe3 + emission if direct electron transfer from theCe3 + ground state to theexcited states of Sm2 + is being pumped, or quenches the luminescence if theCe3 + excitedstates are pumped. The experiments allow for a precise measure of the difference in energy between theSm2 + andCe3 + ground states (2.98 eV).