Non-resonant fluorescence

The seed is selectively and sequentially pumped with a light source at two wavelengths and the non-resonant fluorescence is observed in each case. The ratio of the two fluorescence signals is related to the temperature. The method was first demonstrated by Haraguchi, et al. (27), who measured temperatures in a variety of flames and whose work has been extended by... 

The radiating level may often be somewhat below the level to which pumping occurs and the transition between the two levels can be radiationless and take place through heat losses. Also the fluorescence transition sometimes does not end at the ground level. With two levels resonance fluorescence occurs, where stray radiation from the exciting radiation limits the power of detection. In the case of three-level systems there is non-resonant fluorescence, where this limitation does not apply. However, here the radiant densities are much lower. Therefore, populating the excited level will only be sufficiently successful when using very intensive primary sources such as tunable lasers. 

The detection of the fluorescence radiation differs in resonant and in non-resonant AFS. In the first case, the radiation is measured in a direction perpendicular to that of the incident exciting radiation. However the system will suffer from stray radiation and emission of the flame. The latter can be eliminated by using pulsed primary sources and phase-sensitive detection. In the case of non-resonant fluorescence, stray radiation problems are not encountered, although the fluorescence intensities are lower, which necessitates the use of lasers as primary sources and spectral apparatus that will isolate the fluorescence radiation. A set-up for laser excited AFS (Fig. 126) may make use of a pulsed dye laser pumped by an excimer laser. The selection of the excitation line is then done by the choice of the dye and... 

In flame AFS, elements which form thermally stable oxides such as Al, Mg, Nb, Ta, Zr and the rare earths are hampered by insufficient atomization. This is not the case when an ICP is used as the fluorescence volume. Here the detection limits for laser excitation and non-resonant fluorescence are lower than in ICP-AES (Table 18) [663]. ICP-AFS can be performed for both atomic and ionic states [664]. 

Lin CH, Fukii H, Imasaka T, et al. 1991. Synchronous scan luminescence techniques monitoring resonance and non-resonance fluorescence in supersonic jet spectrometry applied to anthracene derivatives. Anal Chem 63(14) 1433-1440. 

The reliability and sensitivity of the probe method has been emphasised. It circumvents almost completely the perturbations inherent to some other probe techniques (electron spin resonance, fluorescence). In particular, free chains appear to be ideal, non-perturbative NMR probes for testing chain segment orientation in strained rubbers. The solvent probe method is easy to handle and unexpensive, since it does not require the synthesis of... 

Oxidation rate constant k, for gas-phase second order rate constants, kon for reaction with OH radical, k os with NO3 radical and ko3 with O3 or as indicated, data at other temperatures see reference koe = (2.4 0.7) X 10 cm molecule" S at 298 K was determined using (flash photolysis-resonance fluorescence, Zetzsch Stuhl 1982 quoted, Atkinson 1985, 1989) k(apparent) < 4 x 10 M s for reaction with ozone at pH 8 in water, k < 6.0 x 10 M" S for protonated species, and k < 6.0 x 10" M" S for non-protonated species for the reaction with ozone in water at pH 2-4 and 20-23°C (Hoigne Bader 1983b)... 

Fig. 3. Transient profile of O ( Pj) density obtained by monitoring the vacuum ultraviolet atomic resonance fluorescence signal following laser photodissociation of O3. (a) Photolysis at 308 nm, Pqs = 1 mTorr, Pjjg = 5.7 Tort, 40288 averaged, (b) Photolysis at 248 nm, Pon = f mTorr, Pjj = 9.7 Torr, 8192 shots averaged. The solid lines through the points representing non-linear least squares fit to the data. [Source Greenblatt and Wiesenfeld, 1983].

The above equations tell us, that the ratio between f disperion and flourescence is proportional to the non-resonant amplitude. Fig. (10) shows that in macromolecular systems the f term may easily exceed fluorescence. 

Cardullo RA, Agrawal S, Flores C, Zamecnik PC, Wolf DE. Detection of nucleic acid hybridization by non-radiative fluorescence resonance energy transfer. Proc. Natl. Acad. Sci. U.S.A. 36. 1988 85 8790-8794. 

Fig. 6. Term schemes for (a) atomic emission, (b) absorption and (c) fluorescence spectrometry (resonant hv and non-resonant hv ).

As especially in the case of non-resonant AFS the fluorescence intensities are low, special precautions have to be taken for signal acquisition. Indeed, the signal-to-noise ratios can be improved considerably by using pulsed signals and phase-sensitive amplification, as is easily done e.g. in the case of laser sources (see Section 2.1). 

Common spectroscopic techniques test small portions of the ground and/or excited state PES either around the gs minimum (IR and non-resonant Raman spectra, electronic absorption spectra.) or in the proximity of the excited state minimum (steady-state fluorescence). These spectra are then satisfactorily described in the best harmonic approximation, a local harmonic approach that approximates the PES with parabolas whose curvatures match the exact curvatures calculated at the specific position of interest [78]. Anharmonicity in this approach manifests itself with the dependence of harmonic frequencies and relaxation energies on the actual nuclear configuration [79]. Along these lines we predicted softened (hardened) vibrational frequencies for the ground (excited) state [74], amplified and p-dependent infrared and Raman intensities [68, 74], different Frank-Condon... 

These contradictory results led Jonkman et al.21 to propose that non-resonant light scattering (NRLS) was responsible for the fast component. NRLS would yield decays that basically consisted of Raman-Rayleigh-scattered laser light together with the slower fluorescence decay. It would look like biexponential decay. Experiments where the laser was purposely detuned from the rotational line seemed to confirm their ideas.21... 

A. Electron-Phonon Interaction Parameterization Scheme. In observing the fluorescence decay rate from a given J-manifold, it is generally found that the decay rate is independent of both the crystal-field level used to excite the system and the level used to monitor the fluorescence decay. This observation indicates that the crystal-field levels within a manifold attain thermal equilibrium within a time short compared to the fluorescence decay time. To obtain this equilibrium, the electronic states must interact with the host lattice which induces transitions between the various crystal-field levels. The interaction responsible for such transitions is the electron-phonon interaction. This interaction produces phonon-induced electric-dipole transitions, phonon side-band structure, and temperature-dependent line widths and fluorescence decay rates. It is also responsible for non-resonant, or more specifically, phonon-assisted energy transfer between both similar and different ions. Studies of these and other dynamic processes have been the focus of most of the spectroscopic studies of the transition metal and lanthanide ions over the past decade. An introduction to the lanthanide work is given by Hiifner (39). 

By covalent attachment of fluorescein (as donor) and rhodamine (as acceptor) to the 5 -termini of complementary oligodeoxyribonucleotides, hybridization can be measured by non-radiative fluorescence resonance energy transfer (FRET) from fluorescein to rhodamine as hybridization occurs, the emission intensity of fluorescein falls while that of rhodamine rises." 9 Correction must be made for... 

Several direct spectrophotometric methods are used for the sulphur dioxide measurement, including non-dispersive infrared absorption, ultraviolet absorption, molecular resonance fluorescence and second-derivative spectrophotometry. 

For small particles, and in many other cases, one will use fluorescence instead of transmission. However, saturation effects such as we saw for transmission mode also occur in fluorescence as well (Troger et al. 1992 Castaner and Prieto 1997). The classic case here is that of a thick piece of pure metal such as Cu. In this material, the ratio of the resonant to non-resonant absorption is about 85 15. This means that for every 100 incident photons, 85 of them create -holes and thus could stimulate fluorescence. Now, suppose the resonant absorption goes up by 10% due to EXAFS. Now the ratio is 93.5 15, or about 86.1 13.9. Thus, the resonant process accounts for 86.1% of the total, which means the fluorescence intensity only goes up by 1.3% instead of 10%. This example shows that, again, the response saturates as a function of the absorption one wants to measure. 

If the resonance detector is well-designed, the vast majority of the magnesium atoms are unexcited. The resonance lines from the magnesium hollow cathode lamp will cause the magnesium atoms in the resonance detector to fluoresce. Some of this fluorescence will fall on a photomultiplier detector placed at right angles to the optical path. The intensity of fluorescence is proportional to the intensity of emission. Non-resonant lines from the lamp or from the flame will have no effect on the resonance detector. Therefore, a system of narrow bandwidth is produced without the requirement of a monochromator. 

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