Luminescent transitions, types

More generally, an Auger transition can be defined as a transition in which energy is transferred from one electronic particle to another in such a way that in the final state the energy of one of the particles lies in a continuum. Auger processes can be classified as intrinsic or extrinsic. The former occur in the pure semiconductor, the latter involve electronic states of impurities like in the example in Figure 4.14. All types of luminescence transitions described in Section 3.3.9 can be quenched by Auger processes. 

The transition type Because luminescence is a technique of electronic spectroscopy, the same selection rules apply for luminescence as those that apply for absorption. For organic compounds, the most common luminescence bands are due to jt - n and jt - n transitions. The a - a transitions, although strongly allowed, are not normally seen because of their high energies in most organic compounds. The spin... 

In this paper we will describe and discuss the metal-to-metal charge-transfer transitions as observed in optical spectroscopy. Their spectroscopic properties are of large importance with regard to photoredox processes [1-4], However, these transitions are also responsible for the color of many inorganic compounds and minerals [5, 6], for different types of processes in semiconductors [7], and for the presence or absence of certain luminescence processes [8]. 

Transitions of this type play a detrimental role in the field of luminescent materials (see also Sect. 7). The Tb(III) and Ce(III) ions in YVO4, for example, spoil all possible luminescence due to the presenee of a MMCT excited state at low energy. This state is ascribed to Tb(III)/Ce(III)-V(V) MMCT resulting in an excited state with character Tb(IV)/Ce(IV)-V(IV) [38, 39]. Very similar are the excited MMCT states in the lanthanide decatungstates LnCWioOjg] " [40] in the cases Ln = Ce, Pr, Tb. Here the excited state has Ln(IV)-W(V) character. 

Interestingly enough, it is possible to study these systems also by emission spectroscopy. The results for In(III) are conspicious (see Table 1). Figure 7 gives the luminescence spectra of LajTaO Clg In(III) to illustrate the type of spectra [48] we are dealing with broad bands the emission is strongly Stokes-shifted relative to the absorption transition. 

This type of centre has also been observed in chlorides [35], Whereas Cs2ZrCl6 and CsjHfClg show blue luminescence due to a CT transition in the ZrClg and HfClg octahedra, the introduction of Pb(II) yields a red luminescence due to MMCT between Pb(II) and Zr(IV)/Hf(IV). 

The introduction of electronic deep levels is demonstrated in Fig. 9 with low-temperature photoluminescence spectra for n-type (P doped, 8 Cl cm) silicon before (control) and after hydrogenation (Johnson et al., 1987a). The spectrum for the control sample is dominated by luminescence peaks that arise from the well-documented annihilation of donor-bound excitons (Dean et al., 1967). After hydrogenation with a remote hydrogen plasma, the spectrum contains several new transitions with the most prominent peaks at approximately 0.95, 0.98, and 1.03 eV. These transitions identify... 

Much of the study of ECL reactions has centered on two areas electron transfer reactions between certain transition metal complexes, and radical ion-annihilation reactions between polyaromatic hydrocarbons. ECL also encompasses the electrochemical generation of conventional chemiluminescence (CL) reactions, such as the electrochemical oxidation of luminol. Cathodic luminescence from oxide-covered valve metal electrodes is also termed ECL in the literature, and has found applications in analytical chemistry. Hence this type of ECL will also be covered here. 

The photoluminescence of these nanoparticles has very different causes, depending on the type of nanomaterial semiconductor QDs luminescence by recombination of excitons, rare-earth doped nanoparticles photoluminescence by atom orbital (AO) transitions within the rare-earth ions acting as luminescent centers, and metallic nanoparticles emit light by various mechanisms. Consequently, the optical properties of luminescent nanoparticles can be very different, depending on the material they consist of. 

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