Luminescent processes

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

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]. 

In radiation chemistry, the track effect is synonymous with LET variation of product yield. Usually, the product measured is a new molecule or a quasi-stable radical, but it can also be an electron that has escaped recombination or a photon emitted in a luminescent process. Here LET implies, by convention, the initial LET, although the actual LET varies along the particle track also, the secondary electrons frequently represent regions of heterogeneous LET against the background of the main particle. 

Ion implantation generates many dangling bonds that form centers for nonradiative recombination. These centers decrease the carrier lifetime and compete effectively with radiative transitions. However, after hydrogenation, since hydrogen ties dangling bonds, the luminescence process becomes more efficient. Furthermore, since the 1.0-eV emission is obtained even before hydrogen is introduced, the new radiative center may be formed due to residual hydrogen in the c-Si that combines with the implantation-induced defects. 

In 1935, after studying the luminescence of various colorants, Jablonski suggested the electronic energy diagram of the singlet and triplet states to explain the luminescence processes of excitation and emission. The proposed diagram of molecular electronic energy levels formed the basis of the theoretical interpretation of all luminescent phenomena [21],... 

When this emission originates from living organisms or from chemical systems derived from them, it is named bioluminescence (BL). Both phenomena are luminescence processes that have been traditionally distinguished from related emissions by a prefix that identifies the energy source responsible for the initiation of emission of electromagnetic radiation. Based on Wiedemann s classification, which was discussed in Chapter 1, contemporary luminescence processes have been added to the list of luminescence phenomena, as can be seen in Table 1. 

In all the luminescent processes, the intensity of the produced emission depends on the efficiency of generating molecules in the excited state, which is represented by the quantum efficiency (quantum yield) and the rate of the reaction. In the case of CL reactions, the intensity can be expressed as ... 

F. Cacialli, R.H. Friend, W.J. Feast, and P.W. Lovenich, Poly(distyrylbenzene- WocA>sexi(ethylene oxide)), a highly luminescent processible derivative of PPV, Chem. Common., 1778-1779, 2001. 

With roughly 1000 atoms, the size of the silicon clusters that constitute the micro PS network is between the bulk crystal and a molecule. Hence models of the luminescence process based on size reduction of the crystal, as well as models based on molecular sttuctures, have been proposed, which are reviewed in detail in [Ca7, Ju3]. Generally the various models of the luminescence of PS can be classified into three major categories ... 

As we will see in the next example, anti-Stokes luminescence is, in general, a nonhnear process that is at variance with the normal luminescence process for which, according to Equation (1.15), the emission intensity is proportional to the light excitation intensity/q. 

Of the many types of bioluminescence in nature, that of the firefly represents the most thoroughly studied and best understood biological luminescent process. The molecular mechanism of light emission by the firefly was elucidated in the 1960s in which a dioxetanone (a-peroxy lactone) was proposed as an intermediate, formed by the luciferase-catalyzed enzymatic oxidation of the firefly luciferin with molecular oxygen (Scheme 15). This biological reaction constitutes one of the most efficient luminescent processes known to date . Hence, it is not surprising that the luciferin-luciferase system finds wide use... 

He speculated that internal conversion was occurring from the 5D3 to the sD4 states and that the luminescence process was step-wise in nature. From his experiments he concluded that to establish thermal equilibrium between the 5Dy and 5D4 states required a time r in excess of 10 3 sec. 

Energy Configuration Diagram. This model, based on the energy level diagrams of atoms and molecules, is applicable to luminescence processes in which excitation and emission take place at the same luminescence center. 

The energy relationships in a luminescence process are presented in a configurational coordinate diagram (Fig. 83). This illustrates the relationship between the potential energy E of the luminescence center (ordinate) and a space coordinate (abscissa), which gives the representative separation between the atom involved and its nearest neighbors or the deflection from its spatial equilibrium position. 

Silver Activation. Doping zinc sulfide with silver leads to the appearance of an intense emission band in the blue region of the spectrum at 440 nm, which has a short decay time. Weak luminescence in the green (520 nm) and red regions can also occur. The blue band is assigned to recombination at substitutionally incorporated silver ions [5.314], [5.315]. The red band is caused by luminescence processes in associates of silver ions occupying zinc positions with neighboring sulfur vacancies... 

Alkali-Metal Halides. Luminescent alkali-metal halides can be produced easily in high-purity and as large single crystals. They are therefore often used as model substances for the investigation of luminescence processes. Their luminescence processes can be divided into 1) the self-luminescence of the undoped crystals, 2) luminescence by lattice defects, and 3) sensitized luminescence. 

The luminescence process itself involves 11) absorption of energy (2) excitation, and tT) emission of energy, usually in the form of radiation in the visible portion of Ihe spectrum. The type of luminescence is usually defined by the excitation means, i.e.. irn/W luminesce nee where excitation is hy cathode rays, as in a television kinescope. The most commonly encountered types of luminescence arc listed in Table I. 

The above properties (a and b) are interpreted by Cohen and Schmidt (21) on the basis of a detailed crystallographic study of photochromic and thermochromic anils. They conclude that photochromic crystals involve structures in which the central portion of adjacent molecules are essentially isolated from one another, so that, to a first approximation the energetics are that of an isolated molecule. On the other hand, when the alignment of the molecular dipoles is such as to give strong intermolecular interactions then the transition to the quinoid form requires much less energy and can occur thermally. For crystals in which thermochromism occurs, the photochemical isomerization is still possible but the reverse reaction is so rapid that no buildup of color is observed. In fact, fluorescence measurements on the thermochromic 5 -chlorosalicylidene-aniline (Fig. 5) indicate that photochemical isomerization precedes the luminescence process via the photochromic route ... 

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