Luminescence principles transitions

Fig. 21. Top The general Jablonski diagram for the flavin chromophore. The given wavelengths for absorption and luminescence represent crude average values derived from the actual spectra shown below. Due to the Franck-Condon principle the maxima of the peak positions generally do not represent so-called 0 — 0 transitions, but transitions between vibrational sublevels of the different electronically excited states (drawn schematically). Bottom Synopsis of spectra representing the different electronic transitions of the flavin nucleus. Differently substituted flavins show slightly modified spectra. Absorption (So- - S2, 345 nm S0 -> Si,450nm 1561) fluorescence (Sj — S0) 530 nm 156)) phosphorescence (Ty Sq, 605 nm 1051) triplet absorption (Tj ->Tn,...

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... 

Luminescence spectra resulting from pure vibrational or V-R transitions involving excited-product states formed in ion-neutral collisions have not yet been observed. However, vibrational and rotational excitation of the products of reactive ion-neutral collisions may be determined indirectly from measurements of Q, the translational exoergicity, which is defined as the difference between the translational energy of the products and that of the reactants. According to the energy-conservation principle, then,... 

Excitation of the Lnm ion by a d-transition metal ion is an alternative to chromophore-substituted ligands, and proof of principle has been demonstrated for several systems. The lack of quantitative data, however does not allow an evaluation of their real potential, except for their main advantage, which is the control of the luminescent properties of the 4f-metal ion by directional energy transfer. In this context, we note the emergence of self-assembly processes to build new edifices, particularly bi-metallic edifices, by the simultaneous recognition of two metal ions. This relatively unexplored area has already resulted in the design of edifices in which the rate of population, and therefore the apparent lifetime, of a 4f-excited state can be fine-tuned by energy transfer from a d-transition metal ion (Torelli et al., 2005). 

Figure 16.4 Principle of the PCT (photoinduced charge transfer), chemically driven, luminescent molecular sensor based on the donor-spacer-acceptor architecture, (a) Binding of analyte trigger to the donor (green) moiety results in hypsochromic shift of absorption (emission) band (b) binding of the same analyte to the acceptor moiety (red) results in bathochro-mic shift of corresponding transition...

Asymmetric Synthesis by Homogeneous Catalysis Coordination Chemistry History Coordination Organometallic Chemistry Principles Dihydrogen Complexes Related Sigma Complexes Electron Transfer in Coordination Compounds Electron Transfer Reactions Theory Heterogeneous Catalysis by Metals Hydride Complexes of the Transition Metals Euminescence Luminescence Behavior Photochemistry of Organotransition Metal Compounds Photochemistry of Transition Metal Complexes Ruthenium Organometallic Chemistry. 

In principle it is necessary to study the lattice dynamics of the whole crystal in order to understand the vibronic features in the luminescence spectra of a centre in a crystal. However, due to the fact that the uranate group can be regarded as an isolated unit in the NaF crystal, the vibronic features in the luminescence spectra of the octahedral uranate group will be mainly due to localized vibrations, and as a starting point we will discuss the luminescence spectra in this section in the site model For an A2 A) transition vibronic coupUng occurs only with the E components of the r 3, r 4, Ps and vibrational modes. For an E A x transition vibronic coupUng might be observed with all the vibrational modes of the UOg complex. 

In GaP, selected pair luminescence (SPL), whose principle is explained in Sect. 1.3.3, has been used by Street and Senske [163] to directly measure the transition energies of the MgGa, Znoa, and Cp acceptors. The advantage of this method is that a value of the ground state energy can also be obtained directly. The absorption by the classical method of a few lines of Be, Mg, Cd, and C acceptors in GaP has also been reported by Kopylov and Pikhtin [103]. 

Since bulk CdS shows free-exciton emission at low temperatures [34], it is interesting to compare these results on CdS with the discussion in Sect. 3.3.9b on the transition from semiconductors to insulators. There it was shown that narrow-line free-exciton emission transforms into broad-band localized emission, if the amount of delocalization of the excited state decreases. Since the valence band to conduction band transition of CdS is in principle a - Cd " " charge-transfer transition, this would bring the discussion on CdS in line with results from a different origin (see Sect. 3.3.9b). By all means the case of Cd32S 4(SC(,Hs)35.DMF4 is a nice example of luminescence research on a well-defined cluster showing the quantum-size effect. 

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