Excited states of bound excitons in ZnO

The bound exciton lines I₅, I₆, and I_{$\text{7}\text{8}$} in ZnO at 5 K are studied in the energetic region of the free excitons and below by means of excitation spectroscopy. Apart from the maxima at the free exciton energies, additional resonances are observed and interpreted as the direct creation of bound excitons in excited states which lie 6meV and 4.5 meV above the ground states.