OPTICAL-EXCITATION OF TRAPPED CHARGES IN QUARTZ, POTASSIUM FELDSPARS AND MIXED SILICATES - THE DEPENDENCE OF PHOTON ENERGY

Optical excitation of trapped charges has been studied by stimulating them with photons of energies from 1.8 to 2.7 eV and measuring the resulting luminescence at higher photon energies. The samples studied were quartz, potassium feldspar and fine-grained silicate extracts from natural sediments. The initial luminescence intensity and its decay as the traps are emptied have been used to gain information about the traps contributing to the emission. For the quartz studied the data are well explained by excitation from a single trap. The excitation spectra for potassium feldspars and mixed silicates show evidence of two different traps, the deepest of which is seen to give rise to the most luminescence. Its optical trap-depth may be more than 2.4 eV while the depth of the shallow trap appears to be about 2.1 eV. In nature, the deeper trap will probably not be well bleached during deposition in all aqueous environments, and for optical dating of water-laid sediments selective sampling of the shallow trap photons with energies less than 2.3 eV for stimulation may prove beneficial.