Optical center

Phosphors usually contain activator ions in addition to the host material. These ions are dehberately added in the proper proportion during the synthesis. The activators and their surrounding ions form the active optical centers. Table 1 Hsts some commonly used activator ions. Some soflds, made up of complexes such as calcium tungstate [7790-75-2] CaWO, are self-activated. Also in many photolurninescence phosphors, the primary activator does not efficiently absorb the exciting radiation and a second impurity ion is introduced known as the sensitizer. The sensitizer, which is an activator ion itself, absorbs the exciting radiation and transfers this energy to the primary activator. 

This work was supported in parts by grants from the State of Utah (Biomedical Optics Center of Excellence grant), by Spectrotek L.C., the National Eye Institute (EY 11600), and the Research to Prevent Blindness Foundation (New York). 

Increasing use is being made of pyran syntheses based upon [4 + 2] cycloadditions of carbonyl compounds. The appropriate unsaturated aldehyde with ethyl vinyl ether yields 53 with peracids this affords an epoxide that undergoes ring contraction to the aldehyde 54 (Scheme 23) and rhodium catalyzed decarbonylation affords the required 3-alkylfuran with the optical center intact.116 Acetoxybutadiene derivatives add active carbonyl compounds giving pyrans that contract under the influence of acids to give... 

Most optical centers show luminescence decay times in the nanoseconds-milliseconds range. However, many other physical processes involved in optical spectroscopy are produced in the picoseconds-femtoseconds range, and mnch more complicated instrumentation becomes necessary. For instance, interband Inminescence in solids, which is of particular interest in semiconductors, can involve decay times in the range of picoseconds. Pulses generated from solid state lasers have already reached this femtosecond domain. 

The corresponding quantum mechanical expression of s op in Equation (4.19) is similar except for the quantity Nj, which is replaced by Nfj. However, the physical meaning of some terms are quite different coj represents the frequency corresponding to a transition between two electronic states of the atom separated by an energy Ticoj, and fj is a dimensionless quantity (called the oscillator strength and formally defined in the next chapter, in Section 5.3) related to the quantum probability for this transition, satisfying Jfj fj = l- At this point, it is important to mention that the multiple resonant frequencies coj could be related to multiple valence band to conduction band singularities (transitions), or to transitions due to optical centers. This model does not differentiate between these possible processes it only relates the multiple resonances to different resonance frequencies. 

Figure 5.1 A scheme of an illustxative optical center, ABg. This particular center consists of a dopant optical ion A in an octahedral environment of B ions.

Molecular orbital theory is a semi-empirical method devoted to interpreting the energy-level structure of optical centers where the valence electron cannot be considered as belonging to a specific ion. In our ABe reference center, this would mean that the valence electrons are shared by A and B ions. The approach is based on the calculation of molecular orbitals (MO) of the ABe pseudo-molecule, V mo, from various trial combinations of the individual atomic orbitals, V a and of the A and B ions, respectively. The molecular orbitals V mo of the center ABe are conveniently written in the form... 

In the previous sections, we have considered that the optical center is embedded in a static lattice. In our reference model center ABe (see Figure 5.1), this means that the A and B ions are fixed at equilibrium positions. However, in a real crystal, our center is part of a vibrating lattice and so the environment of A is not static but dynamic. Moreover, the A ion can participate in the possible collective modes of lattice vibrations. 

Finally, in the last section of this chapter (Section 6.6), we will treat two aspects that are of great relevance in the optical spectroscopy of solids. First, we will introduce a semi-empirical method (due to Judd, 1962 Ofelt,1962) that analyzes the absorption spectra of trivalent rare earth ions in crystals to search for new efficient phosphors and solid state lasers. Secondly, we will treat a relatively new topic related to optical centers in solids the optically induced cooling of trivalent ytterbium doped solids. 

Group theory can also be applied to determine whether an optical transition is allowed in a particular optical center. As we showed in Section 5.3, the probability of a radiative transition between two given states, (initial) and (final), is proportional to... 

At this point in the chapter, we know how to apply group theory to solve the problem of labeling the energy levels of an optical center as well as to determine which transitions are optically allowed. The next few examples are devoted to practice with these aspects. 

Decene complexes with gold, 12 348 Deformation density, 27 29-33 Degradation reactions, heteronuclear gold cluster compounds, 39 336-337 Dehydration reactions, osmium(II), 37 351 Delocalization, see also Valence delocalization added electron, reduced dimer, 38 447, 449 optical centers, interaction with surroundings, 35 380 Density... 

HS redox couple, 33 91-92 HSe/HSe redox couple, 33 98 HSO," /HSO4 redox couple, 33 96 HSO5/HSO5" redox couple, 33 96 [HTcCpJ, 41 29 Huang-Rhys parameter, 35 325 optical centers, interaction with surroundings, 35 380-381... 

ON(SO,)j /0N(S03)2 - redox couple, 33 106 O—O bond, copper proteins, 39 26 homolytic cleavage, 39 60, 62-63 Opposite-spin correlation, 38 439-440 Optical absorption spectrum cytochrome b, 36 418, 420 holoferritin, 36 418-419 Optical centers, interaction with surroundings, 35 319-322... 

The mechanisms of luminescence decay from an optical center are of critical importance. In particular we have to know if there are any processes internal to the center or external to it, which reduce the luminescence efficiency. It is possible to define two decay times, ir, the true radiative decay time which a transition would have in absence of all non-radiative processes, and r, the actual observed decay time, which maybe temperature dependent, as will usually occur when there are internal non-radiative channels, and which may also be specimen dependent, as when there is energy transfer to other impurities in the mineral. The quantum yield may be close to unity if the radiationless decay rate is much smaller than the radiative decay. 

As the beam leaves the prism predisperser, it is focused on the entrance slit of the grating monochromator. The slit is curved, has variable width, and opens symmetrically about the chief ray (optical center line of system). The monochromator itself is of the off-axis Littrow variety (James and Sternberg, 1969 Stewart, 1970 Jennings, 1974) and uses a double-pass system described by McCubbin (1961). The double-pass aspect of the system doubles the optical retardation of the incident wave front and theoretically doubles the resolution of the instrument. The principal collimating mirror is a 5-m-focal-length, 102-cm-diam parabola. 

Interaction between Optical Centers and Their Surroundings An Inorganic Chemist s Approach... 

In fact, there are other considerations that complicate the compositional issue still further. The ad-variants bear a further optically active center as a result of the chain-branch position, which is likely to be racemic (it is adjacent to a carbonyl moiety). Because it is remote through space from other optical centers in a-acids and other optically active hop-derived components, it is unlikely to have a practical bearing on the properties and therefore the application of these compounds. More relevant though is the observation of minor components of the a-acids that have both shorter and longer side chains than the more abundant co-, n-, and ad-variants. Given that hydrophobicity is related to the potency of the brewing value of the hop-derived components, there is justification for the quantification of particularly the more hydrophobic species, as recently exemplified by Wilson et al. (18). 

In this communication we will give a description of the vibronic E-e interaction in an optical center in a crystal near one of the minima of the trough of the deformed (due to the quadratic vibronic coupling) Mexican-hat-type AP. We will also present a derivation of the nonperturbative formula describing the temperature dependence of the ZPL in the case of an arbitrary change of the elastic springs on the electronic transition. Then we will study a case when the excited state is close to the dynamical instability. Finally, we will apply the obtained general results to the ZPLs in N-V centers in diamond. 

Here we consider an optical transition between Aj and E electronic states of a center of a trigonal symmetry. To describe the vibrations of the center we use the collinear-configurational approximation [27] in which only the central forces are taken into account in the optical center (taking account of deviations from this approximation, see later). If one restricts oneself to the linear vibronic coupling in the e state, then in this approximation the potential energy operators in the Ai and E electronic states can be presented in the form ... 

From the physical consideration it is clear that in the wt = 0 limit the optical center in the initial state is just on the verge of the dynamical instability. This means that the relation wiGo(O) = 1 holds for the model under consideration. Besides, if c = 0 then G2(a>) = G0(excited state is (as it should be) on the verge of the dynamical instability. Consequently in the case of strong linear Jahn-Teller effect at low temperatures the ZPL is described by equations (19) and (20). Consequently the width of the ZPL in the case of the optical transition Ai-E with the strong linear Jahn-Teller effect in the E-state increases with temperature as 7 3. 

Although no quantum confinement should occur in the electronic energy level structure of lanthanides in nanoparticles because of the localized 4f electronic states, the optical spectrum and luminescence dynamics of an impurity ion in dielectric nanoparticles can be significantly modified through electron-phonon interaction. Confinement effects on electron-phonon interaction are primarily due to the effect that the phonon density of states (PDOS) in a nanocrystal is discrete and therefore the low-energy acoustic phonon modes are cut off. As a consequence of the PDOS modification, luminescence dynamics of optical centers in nanoparticles, particularly, the nonradiative relaxation of ions from the electronically excited states, are expected to behave differently from that in bulk materials. 

Nowadays the core-shell (or nanocoating) technique is extensively applied to the synthesis of both semiconductor and insulating nanostructures for a variety of purposes. By modification of their nanostructure or surface, one can improve the quantum efficiency of lanthanide optical centers, and design biolabels. In addition, core-shell particles can be used as precursor to produce hollow spheres. 

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