Fluorescence scattering experiments

The hydroxyl concentration profile for a stoichiometric CH -air flame is presented in Figure 8. Here the maximum mole fraction observed and the predicted mole fraction are equal to better than 10% accuracy. The abscissas of the theoretical and the experimental results were matched by setting the theoretically predicted temperature equal to the measured hydroxyl rotational temperature. At all positions in the flame the hydroxyl 2j[(v,=o) state exhibited a Boltzmann distribution of rotational states. This rotational temperature is equal to the N2 vibrational temperature to within the +100 K precision of the laser induced fluorescence and laser Raman scattering experiments. An example of this comparison is given in Figure 9. 

Figure 4.7 (A) Schematic illustration of fluorescence enhancement experiment Ag nanoprisms are adsorbed on top of monolayer of Rhodamine red on glass slide. (B) Darkfield scattering image of an area of the substrate. Each of colored...

The techniques of optical absorption and fluorescence are not suitable in the study of metals and intermetallics because of the weak intensity of transitions and the inability of radiation to penetrate the metal. However, neutron inelastic scattering experi-... 

Fig. 8. Interstitial velocity profiles. Representative regions in the microcirculation. Circles represent locations of fluorescence photobleaching experiments. The arrows inside the circles represent the direction of the interstitial fluid velocity at these locations. The nearby values show magnitudes of the velocity in fim/s. (a) An area where interstitial flow parallels blood flow in the vessels, (b) Interstitial flow is opposite prevailing blood flow, (c) Fluid is absorbed from the interstitium into a postcapillary venule. (From Chary and Jain, 1989, with permission.) The photobleaching technique has provided the first and to date the only measurements in the literature of interstitial convective velocities. We have now further improved this technique to permit measurements of binding parameters (Kaufman and Jain, 1990, 1991, 1992a, b) and of transport parameters in light-scattering media (Berk et al., 1993).

This chapter can be regarded as optional since (a) it is not necessary for the logical development of the subject and (b) no light-scattering experiment has yet been performed which unambiguously gives rate constants. The reader s attention is directed to Section (6.6) for a discussion of Fluorescence Fluctuation Spectroscopy which has been successfully used to determine rate constants. 

Thus, if ti is in a time range accessible to autocorrelation (roughly 1—10—7 sec), fluorescence fluctuations may be used to measure macromolecular translational diffusion coefficients. The presence of a fluorescent label enables this method to measure the translational diffusion coefficient of a molecule in a complex mixture. Such a measurement would be very difficult in an ordinary light-scattering experiment because all components of the mixture contribute.17 The advantage of fluorescent probes is that they allow particular species to be labeled and thereby separately studied. For or of the order of 10 4 cm and for a particle with a diffusion coefficient of the order 10 5 cm2/sec, tt = 10 3 sec, well within our ability to measure. This leads to the interesting possibility of measuring diffusion coefficients of labeled molecules in membranes, and in cells in vivo. 

The experimental station contains manipulators to positions the sample, optics to view the sample, and detectors to measure the fluorescent, scattered or diffracted X-rays. The details of these apparatus depend entirely on the type of experiment to be performed. For example, an XRF microprobe requires a precision X-Y-Z sample stage, high quality microscope and multi-element fluorescence detector. A surface scattering experiment, on the other hand requires a 4-circle goniometer and a low-noise photon counting detector. 

For light scattering experiments, the choice of the laser is critical. Natural organics can be fluorescent compounds when excited by lower laser wavelengths. This interferes with measurements. This effect can be minimised by the choice of a red laser. Additionally, given the low concentrations required to avoid molecule interactions, aggregation or multiple scattering of these small compounds, a very powerful laser is required. 

Nondescanned (or direct ) detection solves a problem endemic to fluorescence scattering in deep sample layers. Fluorescence photons have a shorter wavelength than the excitation photons and experience stronger scattering. Photons from deep sample layers therefore emerge from a relatively large area of the sample surface. To make matters worse, the surface is out of the focus of the objective tens. Therefore the fluorescence cannot be focused into a pinhole. 

Further progress is expected from time-resolved separations of competing inter-and intra-molecular processes, from state-resolved fluorescence experiments with polyatomic molecules, or from inelastic scattering experiments of highly excited polyatomic molecules. 

In Table 1 we present the Zp values determined in THF and two different THE/DME mixtures. These values, on the order of 1 rm, are comparable to those reported by Discher and coworkers [38,70] and by Bates and coworkers [71,72] for PEO-PI cylindrical micelles with a core diameter of 20 nm in water. Here PEG denotes poly(ethylene oxide). Bates and coworkers deduced their values of Zp from small-angle neutron scattering experiments, whereas Discher and coworkers determined the Zp values using fluorescence microscopy. The fact that the Zp values that we determined from viscometry are comparable to those of the PEO-PI cyUndrical micelles with similar core diameters again suggests the validity of the YFY theory in treating the nanofiber viscosity data. This study demonstrates that block copolymer nanofibers have dilute solution properties similar to those of semi-flexible polymer chains. 

Fluorescence-sensitization experiments have been made with molecular crystals containing a small amount of an impurity with lower excitation energy. The best known example is that of anthracene crystals with traces of naphthacene . These crystals emit the naphthacene fluorescence even at an impurity mole ratio of 10 with exclusive excitation of the host molecules. Here the multi-step nature of the transfer process is evident because of the weak coupling properties of the host material. It is further substantiated by the low concentration of the acceptor required and by the decrease in transfer efficiency at low temperature . The transfer is expected here to occur by the migration of vibronic excitons, which, after more or less efficient scattering, are finally trapped by an impurity molecule with its lower energy state. 

According to Fig. 20 and the pertinent discussion, besides the light-shifted absorption line also an ampUfication hne should appear for a non-resonant pump beam. Actually, in the single molecule fluorescence excitation experiments population changes are measured and the amplification effect is expected to be very weak. Due to the large scattering background induced by the pump it was not observed so far. 

Transform limited pulses of a duration of 9 ns with a very stable center frequency were generated as follows A tunable single mode dye laser with a bandwidth of 2 MHz was pulsed with the help of an acousto-optical modulator. For the fluorescence lifetime experiments pulses of 9 ns FWHM and a separation of 1 ps were used. The laser pulses were focused onto a pinhole with a diameter of 5 pm and illuminated about 100 pm of the sample crystal. A set of lenses imaged the emerging fluorescence onto a fast photomultiplier. Optical cutoff filters removed scattered laser light from fluorescence. Recording times up to 1200 s were necessary to obtain reliable statistics for the fluorescence decay curves. The laser had to be stabilized onto a fixed frequency during the full measurement time. 

In a second study, they evaluated the interfacial thickness of twopoly(isoprene-b-methyl methacrylate) block copolymers (Pl-PMMA) using the same approach. Small-angle X-ray scattering experiments showed that films of the mixed diblock copolymers have a lamellar morphology with a spacing that varies with composition from 24 to 26 nm. Fluorescence decay profiles from these films were analyzed in terms of an energy transfer model that takes into account the distribution of junctions across the interface and calculated an interface thickness of 1.6 + — 0.1 nm. This value was independent of the acceptor/donor ratio (i.e., the acceptor concentration) in the films. 

Electroreflectance, fluorescence and elastic light-scattering experiments used to study the mechanism by which surfactants spread at the MS interface are described in [25]. Monochromatic linearly polarized light was focused onto the electrode surface at a 45° angle. The specularly reflected light was collected in electroreflectance experiments. Fluorescence and elastically scattered light were collected at an angle of 90° with respect to the plane of incidence and 45° with respect to the electrode surface. 

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