A detailed investigation has been made of the photoconductivity response of several types of natural diamond in the spectral energy range from 3.0 to 5.9 eV. Oscillatory structure in the photoconductivity beyond the fundamental absorption edge at 5.49 eV has been shown to be closely related to similar oscillatory structure in the luminescence-excitation spectra, but does not appear in the absorption spectrum. The oscillations appear only together with the N9 absorption/luminescence system with no-phonon lines at 5.25 and 5.26 eV, which has recently been attributed to the creation and decay of indirect excitons bound to nearest-neighbor nitrogen-aluminum donor-acceptor pairs (5DA complex). The photoconductivity minima and the luminescence-excitation maxima are attributed to the enhanced probability of the creation of bound-exciton N9 states if the residual kinetic energy of free excitons is very low after the dissipation of the majority of their excess energy through the cascade emission of optical phonons. Photoconductivity quenching has also been observed in the region of the no-phonon lines of the N9 system, and the temperature dependence of this effect has been found to be consistent with one of the proposed decay modes of the 5DA complex. A photoconductivity threshold has been observed at 4.05 eV in all insulating diamonds, which is considered to be the photoionization limit for the isolated substitutional nitrogen donor. Photo-Hall-effect measurements have been made which support this explanation.

Some optical, dielectric and lattice properties of three alkaline earth fluorides (calcium, strontium and barium fluorides) and of two other isomorphous fluorides (cadmium and lead fluoride) are reported. Results obtained from experimental studies on infrared and Raman spectra give the fundamental transverse and longitudinal optical lattice mode frequencies, the two-phonon absorption spectra, and the spectra associated with U centre absorptions. Calculations of phonon dispersion curves, density of states and density of com­bined states functions, have been made for all five crystals and these are compared with the experimental results.

No abstract

Paul Denham has died at the early age of 46. He studied physics at King's College London, gaining a first class degree in 1964 and completing his PhD on electroluminescence in diamond in 1967.