Excitonic states in diamond in the spectra of optical absorption and luminescence

The optical absorption and photoluminescence spectra of two synthetic diamonds in the temperature range of 80-300 K were measured. It is established that the free exciton state in a diamond is split by at least 12 sublevels. The "negative" branch of interband transitions makes the main contribution to the temperature dependence of the optical absorption spectrum near the fundamental edge. For the "negative" branch of interband transitions with the transverse optical (TO) phonon absorption, the transition probability falls from 9⋅108 to 1.8⋅105 cm−1⋅eV−1 with an increase in temperature from 83 to 202 K. With temperature increase up to 294 K, the value of transition probability shows practically no changes. For the "positive" branch, the probability of interband transitions with the generation of TO phonons in diamond is 2⋅108 cm−1⋅eV−1 in the temperature range 83-227 K. As the temperature rises from 227 to 294 K, the probability of these transitions increases to 2⋅109 cm−1⋅eV−1. The fine splitting of the free exciton state leads to an additional broadening of the radiative recombination bands of free excitons in diamond.