A density of states diagram for the mobility gap of a-Se is constructed by self-consistent analyses of time-resolved xerographic residual and dark depletion potentials in both undoped and halogen- and alkali-doped Se films. These are then combined with analyses of transient transport data to reveal additional features adjacent to the mobility edges. The results are consistent with the suggested existence of oppositely charged diamagnetic defect pairs whose relative population can be significantly and systematically altered by association with halogens and alkalis.