Excited states of excitons bound to nitrogen pairs in GaP

We report the luminescence excitation spectra of bound excitations in GaP:N below 2 K. A dye laser was used as a tunable excitation source. The spectra of excitons bound to $\mathrm{N}{\mathrm{N}}_i$ pairs and to single N traps reveal a series of sharp transitions to excited states for those pairs having total exciton binding energy of more than 40 meV. For the shallower $\mathrm{N}{\mathrm{N}}_i$ traps ($i\ge 7$) and the single N, the excited states overlap the free-exciton continuum and are broad. The sharp excited states are identified as $\mathrm{nS}$ states of an acceptorlike series resulting from the motion of a hole in the Coulomb field of the tightly bound electron. The dependence of the excited-state energies on the principal quantum number $n$ agrees very well with the effective-mass calculations of Baldereschi and Lipari. The observed core correction is negative, contrary to the case of normal acceptors. These observations provide direct proof of the validity of the Hopfield-Thomas-Lynch model for isoelectronic traps. Besides these acceptorlike excited hole states, we find evidence for a possible excited state of the electron bound to N${\mathrm{N}}_1$, 85 meV above the ground state.