Transition emission spectra from

Emission Spectra, The emission spectra from transition metal complexes can be an extremely useful aid in studying molecular electronic states. The binuclear complex Cu2(PAA)2en has a sharp fluorescence emission at 427 nm. This is unusual since Cu(II) compounds rarely, if ever, are observed to emit. It is significant that neither Cu(Acac)2 nor Cu(Acac)2Cn emits under the experimental conditions used to study Cu2(PAA)2en. Indeed, there are no reports in the literature of emission from either of these chelates under any experimental conditions. The fluorescence spectra of the protonated ligand, H4(PAA)2en, and of Cu2(PAA)2en are compared in Table IV. The excitation wavelength in... 

The spectrum of atomic hydrogen, as observed in absorption or emission, arises from transitions between tile various possible states. In emission, a spectral line results from a transition such as n2 i and the application of Eq. (84) leads to the expression... 

The radiative branching ratio can be calculated through the probabilities of the transitions, or, in the ease of emission, it can be determined experimentally from the emission spectra, where E JJ ) is the integrated emission spectrum of transition and J ) is the... 

Section 6.13.2 and illustrated in Figure 6.5. The possible inaccuracies of the method were made clear and it was stressed that these are reduced by obtaining term values near to the dissociation limit. Whether this can be done depends very much on the relative dispositions of the various potential curves in a particular molecule and whether electronic transitions between them are allowed. How many ground state vibrational term values can be obtained from an emission spectrum is determined by the Franck-Condon principle. If r c r" then progressions in emission are very short and few term values result but if r is very different from r", as in the A U — system of carbon monoxide discussed in Section 7.2.5.4, long progressions are observed in emission and a more accurate value of Dq can be obtained. 

The resulting PL intensity depends on the absorption of the incident light and the mechanism of coupling between the initial excited states and the relaxed excited states that take part in emission. The spectrum is similar to an absorption spectrum and is useful because it includes higher excited levels that normally do not appear in the thermalized PL emission spectra. Some transitions are apparent in PLE spectra from thin layers that would only be seen in absorption data if the sample thickness were orders of magnitude greater. 

Fig. 74 shows the IR emission spectrum of the transition of K2TaF7 from solid to molten state. 

Since an atom of a given element gives rise to a definite, characteristic line spectrum, it follows that there are different excitation states associated with different elements. The consequent emission spectra involve not only transitions from excited states to the ground state, e.g. E3 to E0, E2 to E0 (indicated by the full lines in Fig. 21.2), but also transisions such as E3 to E2, E3 to 1( etc. (indicated by the broken lines). Thus it follows that the emission spectrum of a given element may be quite complex. In theory it is also possible for absorption of radiation by already excited states to occur, e.g. E, to 2, E2 to E3, etc., but in practice the ratio of excited to ground state atoms is extremely small,... 

Fluorescent probes are divided in two categories, i.e., intrinsic and extrinsic probes. Tryptophan is the most widely used intrinsic probe. The absorption spectrum, centered at 280 nm, displays two overlapping absorbance transitions. In contrast, the fluorescence emission spectrum is broad and is characterized by a large Stokes shift, which varies with the polarity of the environment. The fluorescence emission peak is at about 350 nm in water but the peak shifts to about 315 nm in nonpolar media, such as within the hydrophobic core of folded proteins. Vitamin A, located in milk fat globules, may be used as an intrinsic probe to follow, for example, the changes of triglyceride physical state as a function of temperature [20]. Extrinsic probes are used to characterize molecular events when intrinsic fluorophores are absent or are so numerous that the interpretation of the data becomes ambiguous. Extrinsic probes may also be used to obtain additional or complementary information from a specific macromolecular domain or from an oil water interface. 

The absorption and emission spectra of a fluorophore are bands spread over a range of wavelengths with at least one peak of maximal absorbance and emission that corresponds to the So-Si and Si—S0 transitions, respectively. There are several vibrational levels within an electronic state and transitions from one electronic to several vibrational states are potentially possible. This determines that the spectra are not sharp but consist of broad bands. The emission spectrum is independent of the excitation wavelength. The energy used to excite the fluorophore to higher electronic and vibrational levels is very rapidly dissipated, sending the fluorophore to the lowest vibrational level of the first electronic excited state (Si) from where the main fluorescent transition occurs [3] (see Fig. 6.1). 

As a result of the atomic nature of the core orbitals, the structure and width of the features in an X-ray emission spectrum reflect the density of states in the valence band from which the transition originates. Also as a result of the atomic nature of the core orbitals, the selection rules governing the X-ray emission are those appropriate to atomic spectroscopy, more especially the orbital angular momentum selection rule A1 = + 1. Thus, transitions to the Is band are only allowed from bands corresponding to the p orbitals. 

The idea of a transition between two energy levels suggests that the transition will occur at only one precise frequency as a sharp spike in the absorption or emission spectrum. This is not the case and, in fact, the transitions have an intrinsic width and shape containing information about the local environment of the atoms. The line profile of an atomic transition has contributions from three effects ... 

The emission spectrum of the cathode material includes a number of intense, sharp lines arising from transitions between excited states and the ground state, so-called resonance radiation. Generally, only a few resonance lines per element are suitable for quantitative work and there will be variation in the ranges of concentration over which absorbance measurements... 

The emission spectrum of some PT and PBD polymer bilayer devices cannot be explained by a linear combination of emissions of the components. Thus, white emission of the PLEDs ITO/422/PBD/A1 showed Hof 0.3% at 7 V, and consisted of blue (410 nm), green (530 nm), and red-orange (620 nm) bands. Whereas the first and the last EL peaks are due to the EL from the PBD and the PT layers, respectively, the green emission probably originates from a transition between electronic states in the PBD layer and hole states in the polymer... 

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