Luminescence excitation spectra and recombination radiation of diamond in the fundamental absorption region

Abstract
Luminescence excitation spectra in the quantum energy interval 5.0 to 7.5 eV (2400 to 1600 $\overset{\circ}{\mathrm A}$) have been obtained for some seventy natural diamonds of gem quality. Below 5.55 eV, structure is present which is directly related to certain components of the optical absorption spectrum, but no structure is found which can be related to the characteristic type I ultra-violet absorption. The excitation spectra of semiconducting diamonds contain a strongly temperature-dependent component whose structure corresponds to the initial structure of the indirect absorption edge. In insulating diamonds, this component is relatively weak and is masked by a system of sharp bands, which are also commonly found in the absorption spectrum. All diamonds show excitation thresholds near 5.486 and 5.538 eV at 80 $^\circ$K, corresponding with the low energy thresholds of the relatively temperature-independent part of the absorption spectrum. Above 5.55 eV there is a system of regularly spaced excitation bands which extends to nearly 7.0 eV. In semiconducting diamonds these bands can be observed only at temperatures below about 100 $^\circ$K and experimental difficulties have prevented confirmation of their presence above 6.0 eV. The structure is tentatively interpreted in terms of exciton and free carrier formation together with multiphonon emission processes in the strain field of lattice defects. At temperatures above about 100 $^\circ$K, semiconducting diamonds exhibit a broad excitation band with a maximum near 6.0 eV, which is thought to be associated with direct transitions from acceptor centres to free carrier or exciton states. An interpretation of the spectrum of the intrinsic recombination radiation of diamond is presented in which the observed peaks result from the phonon-assisted recombination of excitons in thermal equilibrium with the lattice.
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