Luminescence spectroscopy experiments

Table XV. Luminescence Spectroscopy Experiments and Miscellaneous Electronic Structure Experiments...

Note that a similar situation arises in the study of heterogeneous deactivation of electron-excited molecules of N2. Thus, an opinion expressed by Clark et al. [152] states that the coefficients of heterogeneous deactivation of N2(A S, v = 0.1) for all surfaces are close to unity. On the other hand, Vidaud with his coworkers [59, 153] have obtained 3 10 2 and (1.8 + 1.2) 10 values for these coefficients shown by platinum and Pyrex, respectively. Tabachnik and Shub [154] investigated heterogeneous decay of NaC A SJJ ) molecules on a quartz surface by the method of bulk-luminescence spectroscopy. The authors carried out a series of experiments within a broad (about four orders of magnitude) range of active particle concentrations and arrived at a conclusion that at a concentration of N2( A 2 ) in excess of 10 mole/cm , the... 

The spectroscopy experiments are further subdivided into atomic spectroscopy found in Table XII, infrared and Raman spectra found in table XIII, visible and ultraviolet absorption spectroscopy found in table XIV, and luminescence spectroscopies found in table XV. 

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

Circularly polarized luminescence spectroscopy (CPLS) is a measure of the chirality of a luminescent excited state. The excitation source can be either a laser or an arc lamp, but it is important that the source of excitation be unpolarized to avoid possible photoselection artifacts. The CPLS experiment produces two... 

Circularly polarized luminescence spectroscopy (CPLS) is a measure of the chirality of a luminescent excited state. The excitation source can be either a laser or an arc lamp, but it is important that the source of excitation be unpolarized to avoid possible photoselection artifacts. The CPLS experiment produces two measurable quantities, which are obtained in arbitrary units and related to the circular polarization condition of the luminescence. It is appropriate to consider CPLS spectroscopy as a technique that combines the selectivity of CD with the sensitivity of luminescence. The major limitation associated with CPLS spectroscopy is that it is confined to emissive molecules only. 

Natural nucleobases are essentially nonemissive with exceedingly low fluorescence quantum yields (f <3 x lO" ) suggesting subpicosecond excited state Ufetimes." Incorporation of nonnatural chromophores into (or on) the bases can provide information about the local environment by monitoring transient or steady-state fluorescent emission (see Luminescent Spectroscopy in Supramolecular Chemistry, Techniques). Emission intensity can increase, decrease, or shift in wavelength depending on the chro-mophore and its local enviromnent, making these fluorescent analogs tunable for specific experiments. Synthetic fluorescent bases can be utilized as probes for nucleic acid structure, dynamics, and interactions. There is an extensive amount of structural variation in fluorescent nucleobase mimics because there is no universal chromophore that is adaptable to every biopolymeric system. 

The octahydrate series divided structurally into two parts (La Ce and Pr Lu, Y) provides an opportunity for comparative thermoanalytical studies in which the effects of crystal structure and central ion size are studied. The octahydrates have been widely investigated by thermoanalytical techniques. The early comprehensive studies by Wendlandt and coworkers using TG and DTA techniques (Wendlandt, 1958 Wendlandt and George, 1961 Nathans and Wendlandt, 1962) have been more recently complemented by the simultaneous TG/ DTA study of Bukovec et al. (1975). These latter authors have also used DSC to determine the dehydration enthalpies. Individual rare earth sulfate hydrates have also frequently been investigated. For example, a recent study employed high-resolution luminescence spectroscopy to monitor the decomposition products of Eu2(S04)3 -SHjO during TG experiments (Brittain, 1983). 

The second application of luminescence spectroscopy in polymer science has been as a tool to study polymer systems themselves. Here a fluorescent or phosphorescent dye is introduced into a polymer environment as a molecular sensor of the environment. One chooses the dye with a knowledge of its spectroscopy in the hopes that changes in its emission spectrum, or, in a pulsed experiment, its emission decay profile, will convey detailed molecular level information about the polymer system itself. These are the experiments which mimic applications of luminescent sensor techniques in biology, where these dyes provide information about hydrophobicity in proteins, local polarity at water-membrane interfaces, distances in antibody-antigen interactions, and a wide variety of other issues concerning system morphology and dynamics. 

ABSTRACT. The cyclization properties of polymer chains serve as a useful vehicle for examining various theoretical predictions about the polymer conformation and dynamics. Cyclization phenomena are particularly sensitive to excluded volume phenomena. This chapter provides a review of pertinent theoretical concepts and a description of experiments, based upon luminescence spectroscopy, which allow one to examine the predictions of the theory. 

Various approaches have been taken toward elucidating morphology and dynamics at this level. Important information has become available frcmi experiments using solid state nmr, electron microscopy, neutron-and x-ray scattering. Luminescence spectroscopy should be able to provide information compl entary to these other techniques. An important consideration is whether these luminescence techniques would be useful in studying systems as complex as typical industrial polymer materials. 

Cathodoluminescence (CL), i.e., the emission of light as the result of electron-beam bombardment, was first reported in the middle of the nineteenth century in experiments in evacuated glass tubes. The tubes were found to emit light when an electron beam (cathode ray) struck the glass, and subsequendy this phenomenon led to the discovery of the electron. Currendy, cathodoluminescence is widely used in cathode-ray tube-based (CRT) instruments (e.g., oscilloscopes, television and computer terminals) and in electron microscope fluorescent screens. With the developments of electron microscopy techniques (see the articles on SEM, STEM and TEM) in the last several decades, CL microscopy and spectroscopy have emerged as powerfirl tools for the microcharacterization of the electronic propenies of luminescent materials, attaining spatial resolutions on the order of 1 pm and less. Major applications of CL analysis techniques include ... 

Experimental technique used during these investigations is usual for Raman scattering and photoluminescence spectroscopy. For luminescence excitation He-Cd, He-Ne, and Ar+ ion lasers were used. The exciting light power not exceeds 25 mW in all experiments. 

Silver halide microcrystals are wide band gap semiconductors which exhibit weak photoconductivity. Early experiments demonstrated that dyes that sensitized silver halide photographic action also sensitized silver halide photoconductivity [6c]. Since the observation of photoconductivity necessitates the movement of free charge within the crystals, dye sensitization processes must inject charge into the silver halide lattice in some way. Initial theories of sensitization were based on the semiconductor view of silver halides, especially as espoused by Gurney and Mott [10]. Current ideas are based on thorough studies of the absorption spectroscopy and luminescence of silver halide emulsions and of adsorbed, sensitizing dyes, and the oxidation-reduction properties of the dyes at silver/silver halide electrodes [11]. 

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