Fluorescence excitation spectrum excited states

Principles and Characteristics The analytical capabilities of the conventional fluorescence (CF) technique (c/r. Chp. 1.4.2) are enhanced by the use of lasers as excitation sources. These allow precise activation of fluorophores with finely tuned laser-induced emission. The laser provides a very selective means of populating excited states and the study of the spectra of radiation emitted as these states decay is generally known as laser-induced fluorescence (LIF, either atomic or molecular fluorescence) [105] or laser-excited atomic fluorescence spectrometry (LEAFS). In LIF an absorption spectrum is obtained by measuring the excitation spectrum for creating fluorescing excited state... 

It is possible that Q is an excited state of Q if so, we will assume that its emission spectrum does not contribute to the fluorescence intensity at Vcnr Q is called a quencher, because in its presence the fluorescence intensity of solute A is reduced. 

Tabic 6-5. Comparison of (he aK vibrational modes in the ground and excited states. The totally symmetric vibrations of the ground stale measured in tire Raman spectrum excited in pre-resonance conditions 3S] and in the fluorescence spectrum ]62 ate compared with the results of ab initio calculations [131- The corresponding vibrations in the excited stale arc measured in die absorption spectrum. 

The fluorescence spectrum of dibenz[7>,/]oxepin shows that this molecule adopts a planar structure in the excited state whereas the ground state has bent geometry as expected.19 The emission spectrum is similar to that of anthracene. 

The simplest fluorescence measurement is that of intensity of emission, and most on-line detectors are restricted to this capability. Fluorescence, however, has been used to measure a number of molecular properties. Shifts in the fluorescence spectrum may indicate changes in the hydrophobicity of the fluorophore environment. The lifetime of a fluorescent state is often related to the mobility of the fluorophore. If a polarized light source is used, the emitted light may retain some degree of polarization. If the molecular rotation is far faster than the lifetime of the excited state, all polarization will be lost. If rotation is slow, however, some polarization may be retained. The polarization can be related to the rate of macromolecular tumbling, which, in turn, is related to the molecular size. Time-resolved and polarized fluorescence detectors require special excitation systems and highly sensitive detection systems and have not been commonly adapted for on-line use. 

The rhodamine B-bound complex of Ir1 (387) shows only minor alterations in the absorption spectrum of bound rhodamine B as opposed to free dye however, its fluorescence is strongly quenched.626 Fluorescence is intense when the rhodamine dye is attached to an Ir111 center. The authors conclude that the excited-state quenching mechanism is via electron transfer. 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

Obviously, a great deal more information could be obtained if the isomeric ions could be probed spectroscopically. Vibrational states of the various isomers are not generally well known, but some structural information is available. Thus, the rotational structure of vibrational transitions may provide a better signature for particular isomers. Certainly, insufficient data are available about the potential surfaces of electronically excited states for electronic excitation to be used as a probe, e.g., as in the very sensitive laser induced fluorescence. At present, there are sensitivity limitations in the infrared region of the spectrum, but this may well be an avenue for the future. The study of isomeric systems and their potential surfaces has just begun ... 

Finally, the third case corresponds to pumping of solutes with 0-0 transitions, Voo>v. In this case, the fluorescence spectrum, immediately after excitation, must be close to the steady state one and should not vary with time. From the physical point of view, this case corresponds to the situation, where solvates with a local field Rtl (corresponding to the equilibrium configuration in the excited state) are excited. 

A number of processes and excited state reactions in the 5) singlet state of organic solutes is possible they may compete with fluorescence and affect directly the quantum yield, the lifetime, and the spectrum of emission. We have reviewed... 

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