Diamond luminescence

Diamond luminescence was studied mainly with the two following aims to carry out a fundamental investigation of its physical properties and to determine the optimal conditions for luminescent sorting of diamond bearing rocks. For the first task, diamond photoluminescence was studied at liquid nitrogen temperature at which luminescence centers are marked by characteristic zero-phonon fines and are much more informative then at room temperature. For the second task, were diamond is one of the first minerals for which luminescence sorting was used, liuninescence properties should be studied at 300 K. In the first stages it was established that X-ray luminescence of the A-band... 

Kimberliteisthe principal diamond bearing ore. In atypical mine such as the Premier Mine near Pretoria, South Africa, one hundred tons of kimberlite produce an average of thirty-two carats of diamond (6.4 g). Dieimonds are sorted from the mineral by an x-ray beam the diamond luminesces with the x-ray and the luminescence activates an air Jet which propels the diamond into a separate bin (Fig. 12.1).i ] Gemstones (a very small percentage) are then separated from the industrial-quality material. 

Diamond luminescence was studied mainly with the two following aims to carry out a fundamental investigation of its physical properties and to determine the... 

The luminescence of diamonds is related to various defects in its structure. Almost always, luminescence centers in diamonds are related to N atoms. It is logical, because the atomic radii of C and N are nearly equal (approximately 0.77 A). Luminescence spectroscopy has proven to be the most widely used method in studies of diamonds even in comparison with optical absorption, ESR, IR and Raman spectroscopies. Himdreds of spectra have been obtained, fluorescence characteristics enter into diamond quality gemological certificates, a wide range of electronic and laser applications are based on diamond optical properties in excited states nitrogen center aggregation is controlled by the residence time of diamond in the mantle, distinction between natural... 

Determination of tint is the essential and the most intricate aspect in diamonds evaluation and may be useful for luminescence interpretation. The coloration of diamond is a reflection of its complex structural pecuharity. To describe a slight shade of color, one has to use a lot of physics, crystallography and analytical tools. The following types of diamond coloration are generally distinguished. 

Fig. 4.71. a-d Laser-induced time-resolved luminescence spectra of diamonds demonstrating N3, A, and H3 centers... 

Fig. 4.72. a-f Laser-induced steady state luminescence spectra of diamonds demonstrating S3, S2, H3, GRl, 700, 788 and possibly 640 nm... 

Ten specimens have been chosen from the collection of approximately 200 characteristic crystals taken from some thousands of Yakutian diamonds. Representative time-resolved luminescence spectra are given in Figs. 4.71-4.72. 

Reabsorption lines of molecular oxygen and water have been detected in the time-resolved luminescence spectra of certain diamonds. 

Our study of time-resolved luminescence of diamonds revealed similar behavior (Panczer et al. 2000). Short-decay spectra usually contain N3 luminescence centers (Fig. 4.71d 5.69a,b) with decay time of r = 30-40 ns. Despite such extremely short decay, sometimes the long-delay spectra of the same samples are characterized by zero-phonon lines, which are very close in energy to those in N3 centers. At 77 K Aex = 308 nm excitation decay curve may be adjusted to a sum of two exponents of ti = 4.2 ps and i2 = 38.7 ps (Fig. 5.69c), while at 300 K only the shorter component remains. Under Aex = 384 nm excitation an even longer decay component of 13 = 870 ps may appear (Fig. 5.69d). The first type of long leaved luminescence may be ascribed to the 2.96 eV center, while the second type of delayed N3 luminescence is ascribed to the presence of two metastable states identified as quarfef levels af fhe N3 cenfer. 

The H3 center is well known in the steady-state luminescence spectra of diamonds. It belongs to the C2v point group, the ground state being level and the excited state from which luminescence takes place a Bi. Both emis-... 

At the present time luminescent sorters are mainly used for the processing of diamonds, but also for the scheehte, fluorite and others types of ores (Mokrousov and lileev 1979 Salter and Wyatt 1991 Gorobets et al. 1997a). The sources of luminescence excitation in these sorters are X-ray tubes and UV lamps, where the first is more powerful and the second is more selective. The UV impulse lasers employment as excitation sources enables us to combine the... 

At the present time new deposits have been discovered which contain diamonds without luminescence under X-ray excitation. Sometimes the portion, which is unrecoverable in X-ray luminescent sorters, is big enough to warrant the development of a new sorting method. The luminescence spectra were investigated under excimer (193,222,248 and 308 nm), nitrogen (337 nm) and dye laser (340-360 nm) excitations (Fig. 8.2). 

ELECTROLUMINESCENCE. Luminescence generated in crystals by electric fields or currents in the absence of bombardment or other means of excitation. It is a solid-state phenomenon involving />- and n-type semiconductors, and is observed in many crystalline substances, especially silicon carbide, zinc sulfide, and gallium arsenide, as well as in silicon, germanium, and diamond. 

Diamond and coworkers attribute an increase in fluorescence from the anthracene reporter sites of 44 to the increased rigidity induced by complexation of Li+, Na+, and K+ to the calixarene s tetraester cleft. The tetraamide derivative, 45, shows an especially selective response to Na+ ion [383], Restricted motion of the calix[4]arene is believed to lead to the enhanced luminescence response. This contention is supported by H NMR studies, which show metal ions to confer significant order on the calix[4]arene receptor. 

Fig. 2. Temperature dependence of the homogeneous width (a) and the peak shift (b) of the 637 nm zero-phonon line in luminescence spectrum of N-V centers in diamond films points experiment the line theoretical approximations according to the laws y — y0 + aT3 + bT1 and 8 = fiT2 - vT4.

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