Luminescence irradiated with electrons

Figure 1. Luminescence of PBD irradiated with electrons at 90 K to a dose of 3.1 eV g". Insert shows spectral distribution of light given...

With small dimensions of the forbidden band the electron transfer of the impurity or of the main substance to the conduction band may take place. The most important luminescent minerals of this kind are ZnS and silver bromides. With the interband spacing of 3-4 eV a UV irradiation with a wavelength of less than 300 nm has enough energy to detach electrons and transfer them from the filled valence band into an empty conduction... 

Two types of Ce centers in calcite were detected by steady-state spectroscopy (Kasyanenko and Matveeva 1987). The first one has two bands at 340 and 370 nm and is connected with electron-hole pair Ce -COj". The second one has a maximum at 380 nm and was ascribed to a complex center with Ce and OH or H2O as charge compensators. Such a center becomes stronger after ionizing irradiation and disappears after thermal treatment. The typical example of Ce luminescence in the time-resolved liuninescence of calcite consists of a narrow band at 357 nm with very short decay time of 30 ns, which is very characteristic for Ce " (Fig. 4.13a). It was found that Ce " excitation bands occurs also in the Mn " " excitation spectrum, demonstrating that energy transfer from Ce to Mn " occurs (Blasse and Aguilar 1984). 

Fig. 5. Energy dependence of the luminescence of CH4 fragments with irradiation by electrons (Reproduced by permission from Ref. 79). Plotted along the axis of ordinates is the intensity of luminescence in arbitrary units.

The nature of the participating defects in EE from LiF has been studied by thermally stimulated luminescence (TSL) ( ). TSL peaks from LiF samples pulverized in a mortar and pestle are quite similar to the peaks from LiF irradiated with UV light, suggesting that the same defects participate in both emissions. TSL studies of MgO powders suggest that above room temperature the rate controlling process involves electron traps. phE and EE decay from MgO follow very similar kinetics, as seen in Figure 1, suggesting that phE and EE are rate limited by the concentration of the same defect. 

In photochemical reduction of CO2 by metal complexes, [Ru(bpy)3] is widely used as a photosensitizer. The luminescent state of [Ru(bpy)3] is reductively quenched by various sacrificial electron donors to produce [Ru(bpy)3] . Metal complexes used as catalyst in the photochemical reduction of CO2 using [Ru(bpy)3] are prerequisites which are reduced at potentials more positive than that of the [Ru(bpy)3] " redox couple (-1.33 V vs SCE) (72). Irradiation with visible light of an aqueous solution containing [Co (Me4(14)-4,ll-dieneNJ], [Ru(bpy)3], and ascorbic acid at pH 4.0 produces CO and H2 with a mole ratio of 0.27 1 (73). Similarly, photochemical reduction of CO2 is catalyzed by the [Ru(bpy)3] /[Ni(cyclam)] system at pH 5.0 and also gives H2 and CO. However, the quantum efficiency of the latter is quite low (0.06% at X = 400 nm), and the catalytic activity for the CO2 reduction decreases to 25% after 4 h irradiation (64, 74, 75). This contrasts with the high activity for the electrochemical reduction of CO2 by [Ni(cyclam)] adsorbed on Hg. 

Cathodoluminescence. Instead of UV radiation, this luminescence is produced by irradiating the sample with electrons (and therefore it usually occurs in a vacuum). This is the phenomenon used in cathode ray tubes (CRTs) (including old TV screens) that were coated on the inside with a ceramic phosphor. 

In the diagram, both defect centers responsible for the UV emission band - 0].j and Oj.j-Va - are presented with ground and excited states forming energy levels between the valence and conduction bands. Irradiation with 245 nm (5.06 eV), corresponding to excitation of the UV band in PL, causes excitation and ionization of Oj.j-v centers. Released electrons move through the conduction band until capture in 0].j and other electron traps. Also direct electron transition onto without involvement of the conduction band is possible. As a result, Oj.j-v centers become acceptors, and Oj., centers - donors, close DAPs with different separation distances are formed. Tunnel transition from the excited state of D to the ground state of A is followed with emission of UV luminescence. 

The Si nanocrystals exhibit photoluminescence upon irradiation with UV light at 230 nm. The MPL spectrum is shown in Figure 10. The spectrum is similar to that reported for 4 nm Si nanocrystals upon excitation with 350 nm at 20 K and also to that PL spectrum of Porous Silicon (49). In these systems the red luminescence is interpreted as a consequence of quantum crystallites which exhibit size-dependent, discrete excited electronic states due to a quantum effect (6,50,51). This quantum confinement shifts the luminescence to higher energy than the bulk crystalline Si (1.1 eV) band gap. This indirect gap transition is dipole forbidden in the infinite preferred crystal due to translational symmetry. By relaxing this symmetry in finite crystallite, the transition can become dipole allowed. As pointed out by Brus (49), the quantum size effect in Si nanocrystals is primarily kinetic mainly due to the isolation of electron-hole pairs from each other. 

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