We study low-strain synthetic high pressure, high temperature diamonds by cathodoluminescence and observe novel fine structure in the free exciton and the boron-bound exciton emission. The basic spectral structure is a doublet with $\Delta E\approx 11\mathrm{meV}$ common to both exciton spectra. This resolves the previously found inequivalence of free exciton ( $\approx 7\mathrm{meV}$) and bound exciton ( $\approx 12\mathrm{meV}$) fine splitting. It is argued that for a spin-orbit interaction ${\Delta }_0$ much smaller than the excitonic binding ( $E_X\approx 80\mathrm{meV}$) and the excitonic localization ( $E_{\mathrm{loc}}\approx 51\mathrm{meV}$) at the boron acceptor, the orbital momentum and the spin of the particles constituting the electron-hole pair are recoupled to form spin singlet and triplet exciton states as the elementary excitations.

• Received 19 October 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.4172