Interference effects, fluorescence

Sichere et al. [25] determined bromine concentrations in the 0.06-120mg/1 range in brines, directly by X-ray fluorescence using selenium as an internal standard to eliminate interference effects. Lower concentrations of bromine must be concentrated on filter paper containing an ion exchange resin. The same concentrations of chlorine can be determined with the addition of barium to reduce the interferences from carbonates and sulfates. Relative standard deviation was better than 1%. The interference of some other ions (e.g., calcium, potassium, magnesium, sodium, and iron) was examined. 

A CCD Raman spectrometer coupled with a 10-mW He-Ne laser has been used to eliminate fluorescence because the long-wavelength excitation by the He-Ne laser is not as likely to cause fluorescent transitions (71). Because of its directional property, coherent anti-Stokes Raman scattering (CARS) is also effective in avoiding fluorescence interference (see CARS in Section 3.9). 

In many respects the selectivities of AFS when an atomic line excitation source is used and AAS should be similar, in so far as both depend upon overlap of extremely narrow absorption and emission line profiles. However, there are differences in the extent of interference effects, even for resonance fluorescence,... 

Each spectroscopic method has a characteristic application. For example, flame photometry is still applicable to the direct determination of Ca and Sr, and to the determination of Li, Rb, Cs and Ba after preconcentration with ion-exchange resin. Fluorimetry provides better sensitivities for Al, Be, Ga and U, although it suffers from severe interference effects. Emission spectrometry, X-ray fluorescence spectrometry and neutron activation analysis allow multielement analysis of solid samples with pretty good sensitivity and precision, and have commonly been applied to the analysis of marine organisms and sediments. Recently, inductively-coupled plasma (ICP)... 

The two SERS bands of eosin at 292 and 500 cm" are developed in spectrum 1 on Fig. 2b. It indicates the major plasmon-dependent mechanism of fluorescence enhancement. Besides the plasmon enhancement, the interference effect between two silver surfaces is possible. We suggest that both these effects are responsible for an observable phenomenon. But more sensitive parameters for the secondary emission enhancement can be tuned through engineering of LP band position and optical density. The low and disadvantageous spectral overlap of LP and molecular absorption bands, as well as the silver deposition excess lead to the significant quenching of analyte fluorescence. 

Another area of interest in quantum interference effects, which has been studied extensively, is the response of a V-type three-level atom to a coherent laser field directly coupled to the decaying transitions. This was studied by Cardimona et al. [36], who found that the system can be driven into a trapping state in which quantum interference prevents any fluorescence from the excited levels, regardless of the intensity of the driving laser. Similar predictions have been reported by Zhou and Swain [5], who have shown that ultrasharp spectral lines can be predicted in the fluorescence spectrum when the dipole moments of the atomic transitions are nearly parallel and the fluorescence can be completely quenched when the dipole moments are exactly parallel. 

The purpose of this section is to discuss the experimental scheme demonstrating the quantum interference effects in the fluorescence intensity and to explore the theoretical approach of Wang et al. [70] that explains the observed intensity profile. 

Measurement of the degree of polarization of the fluorescence from an excited atomic fragment can give more information than f3 about the relative absorption amplitudes for excitation to different dissociative states. The alignment of photofragment emission can give information above the relative phases of the transition moments for photoexcitation of the parent molecule (3 samples only the squared transition moments), due to interference effects (Vigue,... 

Let us now turn our attention to the effect of a thin monolayer of soap molecules present on both film surfaces. In the first place it was found (see Section III.B) by Bouchiat and Langevin that density and orientation fluctuations of the soap molecules (in the case of a single interface) make only a negligible contribution to the scattering process, except possibly at a (two-dimensional) critical point of the monolayer, or by addition of a few fluorescent soap molecules. Thus we are left with the optical effect of the monolayers on the interference of the light waves. It turns out (see Section VI) that this interference effect can be taken into account by redefining the (optical) film thickness. 

Chemical interference effects in atomic fluorescence are similar to those observed in atomic absorption spectroscopy. In addition, any process that affects the quantum efficiency of the fluorescence or disrupts normal emission of the energy of the excited state also can be considered as a chemical interference. 

Another aspect of optical pumping is related to the coherent excitation of two or more molecular levels. This means that the optical excitation produces definite phase relations between the wave functions of these levels. This leads to interference effects, which influence the spatial distribution and the time dependence of the laser-induced fluorescence. This subject of coherent spectroscopy is covered in Chap. 7. 

It should he emphasized that line narrowing is only observed if the time development of the phase of the upper-state wave function can be measured. This is the case for all methods that utilize interference effects caused by the superposition of different spectral components of the fluorescence. Therefore, an interferometer with a spectral resolution better than the natural linewidth can be used, too. However, a narrowing of the observed fluorescence linewidth with increasing gate-delay time T is only observed if the gating device is placed between the interferometer and the detector it is not observed if the gate is placed between the emitting source and the interferometer [1303]. 

P. Franken, Interference effects in the resonance fluorescence of crossed excited states. Phys. Rev. 121, 508 (1961)... 

---