Term analysis, emission spectra

Time-resolved emission spectra were reconstructed from a set of multifrequency phase and modulation traces acquired across the emission spectrum (37). The multifrequency phase and modulation data were modeled with the help of a commercially available global analysis software package (Globals Unlimited). The model which offered the best fits to the data with the least number of fitting parameters was a series of bi-exponential decays in which the individual fluorescence lifetimes were linked across the emission spectrum and the pre-exponential terms were allowed to vary. 

Polymers in Rigid Solution. The emission spectrum of PCVA in 2-methy1tetrahydrofuran (MTHF) at 77 K consists of prominent delayed fluorescence and phosphorescence bands(19). For this reason it was decided to investigate the rate of triplet exciton decay in these rigid solutions and to treat the data in terms of concurrent first and second order processes. For systems in which an equilibrium distribution of potential reactants may be assumed, eq 1 may be employed for data analysis. It is not clear, however, that such a distribution is valid for polymer solutions especially in light of evidence suggesting that T-T annihilations occur principally by intra-coil processes(14-15). 

From the perspective of the atomic spectroscopist, desirable properties of plasmas include high thermal temperature and sufficient energy to excite and ionize atoms which are purposefully introduced for the purposes of analysis. In terms of atomic spectrometry, this means that we would generally wish to measure the absorption or emission of radiation in the near-ultraviolet (180-350 nm) and visible (350-770 nm) parts of the spectrum. In this sense, plasmas have been variously described as electrical flames or partially ionized gases. A working definition for atomic spectrometry could be as follows ... 

Direct labeling of a biomolecule involves the introduction of a covalently linked fluorophore in the nucleic acid sequence or in the amino acid sequence of a protein or antibody. Fluorescein, rhodamine derivatives, the Alexa, and BODIPY dyes (Molecular Probes [92]) as well as the cyanine dyes (Amersham Biosciences [134]) are widely used labels. These probe families show different absorption and emission wavelengths and span the whole visible spectrum (e.g., Alexa Fluor dyes show UV excitation at 350 nm to far red excitation at 633 nm). Furthermore, for differential expression analysis, probe families with similar chemical structures but different spectroscopic properties are desirable, for example the cyanine dyes Cy3 and Cy5 (excitation at 548 and 646 nm, respectively). The design of fluorescent labels is still an active area of research, and various new dyes have been reported that differ in terms of decay times, wavelength, conjugatibility, and quantum yields before and after conjugation [135]. New ruthenium markers have been reported as well [136]. 

The electron affinity can also be deduced from the measurement of the spectrum of the photoelectron emission with monochromatic UV light. This technique is ultra-violet (UV) photoelectron emission spectroscopy (or UV photoemission spectroscopy or UPS). The UPS technique involves directing monochromatic UV light to the sample to excite electrons from the valence band into the conduction band of the semiconductor. Since the process occurs near the surface, electrons excited above the vacuum level can be emitted into vacuum. The energy analysis of the photoemitted electrons is the photoemission spectrum. The process is often described in terms of a three step model [8], The first step is the photoexcitation of the valence band electrons into the conduction band, the second step is the transmission to the surface and the third step is the electron emission at the surface. The technique of UPS is probably most often employed to examine the electronic states near the valence band minimum. 

Emission spectroscopy is the analysis, usually for elemental composition, of the spectrum emitted by a sample at high temperature, or that has been excited by an electric spark or laser. The direct detection and spectroscopic analysis of ambient thermal emission, usually in the infrared or microwave regions, without active excitation, is often termed radiometry. In emission methods the signal intensity is direcdy proportional to the amount of analyte present. 

A spectrometer consists of three main parts (1) an emission source, which produces the spectrum (2) an optical system, which scatters the spectrum and (3) a device to measure the emitted lines. The two major types of instrument for the analysis of emission spectra are sequential and simultaneous spectrometers, although there are many variants of each in terms of mechanical and optical characteristics. The spectral... 

Analysis of the spectrum of the H atom led to the Bohr model, the first step toward our current model of the atom. From its use by 19 -century chemists as a means of identifying elements and compounds, spectrometry has developed into a major tool of modem chemistry. The terms spectmscopy, spectrophotometry, and spectrometry refer to a large group of instrumental techniques that obtain spectra that correspond to a substance s atontic or molecular eneigy levels. (Elements produce lines, but complex molecules produce spectral peaks.) The two types of spectra most often obtained are emission and absorption spectra ... 

The mathematical factor can be understood from the analysis of Eq. 5.1, which states that the number of photons emitted by a sample (/]) is equal to the number of photons absorbed (/J times the emission quantum yield. The number of absorbed photons, resulting from the difference between the number of incident and transmitted photons, is not linearly related to the absorbance, and hence to concentration as shown in the equation, the relationship contains the term [1 — 10 ]. As a matter of fact, this term can be expanded in series, and orders higher than 1 can be neglected if A(Aexc) is reasonably small (usually <0.05 or, if a larger error can be tolerated, <0.1) in other words, the luminescence intensity observed for solutions whose absorbance at Aexc is smaller than 0.05 (or 0.1) is linearly proportional to A(Aexc). This is also the reason why the excitation spectrum of an emitting species, to be quantitatively compared with the absorption spectmm, must be recorded on a solution with very low absorbance (see Sect. 5.3.2). TTie term... 

In steady-state PL, the shape of the spectrum is determined by the level of excitation intensity as the defect-related PL often saturates at power densities on the order of to 10 Wcm, and the overall PL spectrum may be skewed in favor of the excitonic emission at higher excitation densities. Similarly, focusing the laser beam and using small monochromator slit widths would also skew the PL in favor of excitonic transitions. In such a case, the chromatic dispersion of the lenses used to collect the PL, as well as the different effective sizes of the emission spots for the ultraviolet (UV) and visible emission attributed in particular to photon recycling process [24], may lead to a noticeable artificial enhancement of the UV (near band edge) over the visible part in the PL spectrum (mainly defect related). Qualitative terms such as "very intense PL attesting to the high quality of the material are omnipresent in the literature on ZnO. In contrast to the wide use of PL measurements, relatively little effort has been made to estimate the absolute value of the PL intensity or its quantum efficiency (QE) for a quantitative analysis. 

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