Ground-state splitting of the 78-meV double acceptor in GaAs

Infrared Raman spectra of the 78-meV double acceptor in GaAs are reported. These spectra reveal electronic excitations within the threefold-split 1$s^2$ ground-state multiplet of the neutral double acceptor. Based on Raman selection rules the energy-level diagram has been constructed for the ground state with the ${\Gamma }_1$ level lowest in energy and the ${\Gamma }_5$ and ${\Gamma }_3$ levels split apart by 11.8 and 14.9 meV, respectively. The overall ground-state splitting of 14.9 meV is surprisingly large compared to the ionization energy of 78 meV. The assignment for the lowest ground-state level is confirmed by the selection rules measured for the Raman transitions between the 1$s^2$ ${\Gamma }_1$ ground state and the 1s2s excited state.