Energy luminescence spectra

At low temperatures, donors and acceptors remain neutral when they trap an electron hole pair, forming a bound exciton. Bound exciton recombination emits a characteristic luminescence peak, the energy of which is so specific that it can be used to identify the impurities present. Thewalt et al. (1985) measured the luminescence spectrum of Si samples doped by implantation with B, P, In, and T1 before and after hydrogenation. Ion implantation places the acceptors in a well-controlled thin layer that can be rapidly permeated by atomic hydrogen. In contrast, to observe acceptor neutralization by luminescence in bulk-doped Si would require long Hj treatment, since photoluminescence probes deeply below the surface due to the long diffusion length of electrons, holes, and free excitons. 

Luminescence spectrum of scheelite with a broad gate width of 9 ms is shown in Fig. 4.9d. The narrow lines at 490 and 572 nm are usually ascribed to Dy and the lines at 607 and 647 nm to Sm " ". Nevertheless, the relative intensity of the line at 607 nm compared to the line at 647 nm is lower at longer delay times (Fig. 4.9e,f). Besides that with a shorter gate width of 1 ps, when the relative contribution of the short lived centers is bigger, the characteristic lines of Sm " " at 647 nm and Dy " at 575 nm disappear while the lines at 488 and 607 nm remain. Such luminescence is characteristic of Pr ", which was confirmed by a time-resolved luminescence study of scheelite artificially activated by Pr " and Sm " (Fig. 5.5). Unlike in apatite, the phenomenon exists not only under 308 nm, but also under 337 nm excitation due to the higher energy... 

Figures 4.34a,b demonstrate the emission lines of titanite, which according to their spectral positions may be confidently connected with Nd " ". The luminescence spectrum in the 860-940 nm spectral range, corresponding to the transition, contains six peaks at 860, 878, 888, 906, 930 and 942 nm, while around 1,089 nm corresponding to F3/2- fn/2 transition it contains five peaks at 1,047,1,071,1,089,1,115 and 1,131 nm. The decay time of IR luminescence of Nd " equal to approximately 30 ps in titanite is evidently the shortest one in the known systems activated by Nd ". The typical radiative lifetime of this level depends on the properties of the solid matrix and varies from approximately 100 ps to 600 ps (Kaminskii 1996). To explain the fast decay time of Nd " in titanite, the energy level quenching by the host matrix may be considered.

Trivalent samarium activated minerals usually display an intense luminescence spectrum with a distinct hne structure in the red-orange part of the spectrum. The radiating term 65/2 is separated from the nearest lower level 11/2 by an energy interval of 7,500 cm This distance is too large compared to the energy of phonons capable to accomplish an effective non-radiative relaxation of excited levels and these processes do not significantly affect the nature of their spectra in minerals. Thus all detected lines of the Sm " luminescence take place from one excited level and usually are characterized by a long decay time. 

The luminescence spectrum of the Canada apatite contains the yellow band, which is similar to Mn + emission in the Ca(II) site (Fig. 5.71). Nevertheless, this band has short decay time, which is not suitable for strictly forbidden d-d transitions in Mn +. It dominates in the time-resolved spectrum with a delay of 10 ps and gate of 100 ps when the shorter-lived centers are quenched, while the longer-Hved ones are not detected. A change in the lifetime may be indicative of the energy transfer from Mn + by a radiationless mechanism. A condition necessary for this mechanism is coincidence or a close distance between energy level pairs of the ion sensitizer and the ion activator. Here, the process of luminescence is of an additive nature and a longer duration and greater quantum yield of the activator luminescence accompany a reduced... 

In principle such upward or downward transitions can take place between any two energy states. The absorption spectrum of an atom consists of very sharp lines, the frequencies of which correspond to the difference of energies between the two states, E2 — Ex = hv. Similarly the luminescence spectrum of an atom consists of sharp emission lines of the same frequency. Figure 3.3 gives a simple picture of the energy states of an atom and of the transitions which can be observed in the absorption and emission spectra. The... 

Therefore, this low-energy band is assigned to a metal-centered d->p transition instead of as arising from Au---Au interactions. The solid-state luminescence spectrum (Fig. 25) exhibits a phosphorescence emission band centered at 417 nm. This value compares favorably with those reported for solids K[Au(CN)2]58 and Au2(dmb)(CN)2.63... 

Examples of the low temperature luminescence spectra are shown in Fig. 8.12. The luminescence intensity is highest in samples with the lowest defect density and so we concentrate on this material. The role of the defects is discussed in Section 8.4. The luminescence spectrum is featureless and broad, with a peak at 1.3-1.4 eV and a half width of 0.25-0.3 eV. It is generally accepted that the transition is between conduction and valence band tail states, with three main reasons for the assignment. First, the energy is in the correct range for the band tails, as the spectrum lies at the foot of the Urbach tail (Fig. 8.12(6)). Second, the luminescence intensity is highest when the defect density is lowest, so that the luminescence cannot be a transition to a defect. Third, the long recombination decay time indicates that the carriers are in localized rather than extended states (see Section 8.3.3). 

The half width of the luminescence line by the phonon interaction mechanism, from Eq. (8.11), is 2[(2 In 2) ji This is 0.25 eV for the maximum phonon energy of 0.05 eV from the silicon network vibrations, which is a little less than the observed line width. Thus the phonon model indicates that the luminescence spectrum is dominated by the phonon interaction and that the disorder broadening contributes less. 

Cs2ZrCl6. A series of sharp transitions are observed that reflect the undistorted octahedral nature of these excited states. Excitation into these features produces the upconversion luminescence spectrum shown in Fig. 19 a. The 10 K excitation scan of this luminescence is compared to the absorption spectrum in Fig. 19b. The upconversion excitation scan closely follows the absorption profile over the full energy range. This observation leads to the conclusion that at 10 K the dominant mechanism for upconversion in 2.5% Re + Cs2ZrCl6 under these conditions is GSA/ETU. Time-dependent measurements confirm this conclusion (Fig. 19 a, inset), showing the characteristic delayed maximum and a 10 K decay constant (/Cdec = 1400 s ) approximately two times that of the excited... 

The excited-state reduction potential, °( Cr3+/Cr2+), can be estimated using an analysis similar to Hess s law of heat summation (Fig. 8.5). Using the emission maximum (730 nm) in the luminescence spectrum and converting units yields an excited-state energy of 164 kJ mol-1 for [Cr(phen)3]3+. That means that relaxation of the 2E excited state to the ground state involves AG° = 164 kJ mol-1 or a one-electron electro-... 

For the Pt(bphXCH3CN)2 complex [111], the high energy feature of the structured luminescence spectrum is at 493 nm and the luminescence lifetime in acetonitrile at room temperature is 14 ns. The emitting level is thought to be localized on bph. 

Figure 2c shows the near-infrared luminescence spectrum of [Gd(hfac)3NIT-BzImH] compared to its lowest-energy absorption band system. At 5 K, both spectra show well-resolved structure that is similar to the patterns observed for the uncoordinated radical, as summarized in Tables 1 and 2. The corresponding electronic transitions can be observed for many other complexes of lanthanide or d-block metal ions with radical ligands [24-27, 30]. In general, the spectra for lanthanide complexes are very similar to those of the uncoordinated radicals. 

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