Evanescent-wave

These effects have been illustrated by calculations for simple idealized cases. There appear to be quite pronounced effects due to the particle and they must be taken into account in any application of inelastic scattering for quantitative determinations of the amount of a specific molecule in ian aerosol. Model calculations of inelastic scattering by isotropically polarizable electric dipoles uniformly distributed within an otherwise nonabsorbing dielectric sphere exhibit a variation of more than two orders of magnitude in the inelastically scattered intensity per active molecule as the particle size is varied. 

The model of inelastic scattering described in this chapter is based upon the assumption that the active molecule can be represented by a classical oscillating electric dipole whose strength is determined by the strength of the local field at the exciting frequency given by the Lorenz-Mie theory. These assumptions should be reasonable for many molecules embedded in weakly absorbing particles. Recent experiments on fluorescence confirm the qualitative features predicted by the model. 

We gratefully acknowledge the assistance of Dr. Derry Cooke, Dr. Steven Druger, and Mr. Michael Sculley with the numerical calculations. This work was supported in part by DOE Contract No. EE-77-S-02-4361 and NSF Grant CHE 77-13102. 

Van de Hu 1st L ight Scattering by Smalt Fartioles (Wiley, New York 1964) 

Kerker The Scattering of Light and Other Electromagnetic Radiation (Academic Press, New York 1965) 

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The isotropic part has not changed. The quasi pressure (qP) curve splits up into a real and an imaginary branch . During this real part the transversal share of the polarization increases until the wave becomes a quasi shear vertical wave. Furthermore, the wave is not anymore a propagating but an evanescent wave in this part. The branch is again only real, it is part of the quasi shear vertical (qSV) curve of the homogeneous case (dotted line), its polarization is dominated by the transversal share and the wave is a propagating one. For the branches (real) and... 

PTM Photon tunneling microscopy [12] An interface is probed with an evanescent wave produced by internal reflection of the illuminating light Surface structure... 

The dynamics of polymers at surfaces can be studied via dynamic light scattering (DLS), as described in Section IV-3C. A modification of surface DLS using an evanescent wave to probe the solution in a region near the interface has... 

It is interesting to note the analogy of developments in light microscopy during the last few decades. The confocal microscope as a scaiming beam microscope exceeds by far the nomial fluorescence light microscope in resolution and detection level. Very recent advances in evanescent wave and interference microscopy seem to promise to provide even higher resolution (B1.18). 

While the spatial resolution in classical microscopy is limited to approximately X/2, where X is the optical wavelength (tlie so-called Abbe Limit [194], -0.2 pm with visible light), SNOM breaks through this barrier by monitoring the evanescent waves (of high spatial frequency) which arise following interaction with an... 

Figure C 1.5.6. Single Ag nanoparticles imaged with evanescent-wave excitation. (A) Unfiltered photograph showing scattered laser light (514.5 nm) from Ag particles immobilized on a polylysine-coated surface. (B) Bandpass filtered (540-580 nm) photograph taken from a blank Ag colloid sample incubated witli 1 mM NaCl and...

Schuck P 1996 Kinetics of iigand binding to receptors immobiiized in a poiymer matrix, as detected with an evanescent wave biosensor, i. A computer simuiation of the influence of mass transport Biophys. J. 70 1230-49... 

Because the permittivity is negative for cj < transmission through the plasma is cut off and penetration is only by means of evanescent waves. 

The construction of a TXRF system, including X-ray source, energy-dispersive detector and pulse-processing electronics, is similar to that of conventional XRF. The geometrical arrangement must also enable total reflection of a monochromatic primary beam. The totally reflected beam interferes with the incident primary beam. This interference causes the formation of standing waves above the surface of a homogeneous sample, as depicted in Fig. 4.1, or within a multiple-layered sample. Part of the primary beam fades away in an evanescent wave field in the bulk or substrate [4.28],... 

In infra-red attenuated total reflection spectroscopy (IR-ATR) and grazing incidence reflection IR spectroscopy (IR-GIR) the evanescent wave of a totally... 

A very specific surface structure is observed after the annealing of a PS/polybuta-diene (PB) diblock copolymer, PS-b-PB, shown in Fig. 7 b. The surface is very smooth directly after preparation of the film from solution (similar to Fig. 7 a). By annealing at 120 °C the surface structure shown in Fig. 7 b evolves, which we believe is due to the formation of layers of PS and PB parallel to the surface. The outermost layer might not be completely filled due to lack of material leading to steps at the surface. Similar behavior is observed with other diblock copolymers such as PS-b-PMMA [61]. Enrichment of one component is also observed at the surface of a polymer solution [115,116] by X-ray fluorescene and evanescent wave techniques. 

Besides crystalline order and structure, the chain conformation and segment orientation of polymer molecules in the vicinity of the surface are also expected to be modified due to the specific interaction and boundary condition at the surface between polymers and air (Fig. 1 a). According to detailed computer simulations [127, 128], the chain conformation at the free polymer surface is disturbed over a distance corresponding approximately to the radius of gyration of one chain. The chain segments in the outermost layers are expected to be oriented parallel to the surface and chain ends will be enriched at the surface. Experiments on the chain conformation in this region are not available, but might be feasible with evanescent wave techniques described previously. Surface structure on a micrometer scale is observed with IR-ATR techniques [129],... 

Another largely unexplored area is the change of dynamics due to the influence of the surface. The dynamic behavior of a latex suspension as a model system for Brownian particles is determined by photon correlation spectroscopy in evanescent wave geometry [130] and reported to differ strongly from the bulk. Little information is available on surface motion and relaxation phenomena of polymers [10, 131]. The softening at the surface of polymer thin films is measured by a mechanical nano-indentation technique [132], where the applied force and the path during the penetration of a thin needle into the surface is carefully determined. Thus the structure, conformation and dynamics of polymer molecules at the free surface is still very much unexplored and only few specific examples have been reported in the literature. 

At higher ethanol concentrations, ATR spectra should contain the contribution from bnUc species, becanse of the long penetration depth of the evanescent wave, 250 nm. To examine the bulk contribution, the integrated peak intensities of polymer OH peaks of transmission (Ats) and ATR (Aatr) spectra are plotted as a function of the ethanol concentration in Figure 5. The former monitors clnster formation in the bulk liquid, and the latter contains contributions of clusters both on the snrface and in the bulk. A sharp increase is seen in A tr... 

A more direct approach to the photoinduced ET dynamics involves monitoring the lifetime of the excited state at the interface. By illuminating the interface in TIR from the electrolyte phase containing the quencher species, the generation of excited state is limited to the characteristic penetration depth given by the evanescent wave (/ ) [127],... 

ATR is one of the most useful and versatile sampling modes in IR spectroscopy. When radiation is internally reflected at the interface between a high-refractive index ATR crystal (usually Ge, ZnSe, Si, or diamond) and the sample, an evanescent wave penetrates inside the sample to a depth that depends on the wavelength, the refractive indices, and the incidence angle. Because the penetration depth is typically less than 2 pm, ATR provides surface specific information, which can be seen as an advantage or not if surface orientation differs from that of the bulk. It also allows one to study thick samples without preparation and can be used to characterize highly absorbing bands that are saturated in transmission measurements. 

Simply visualised, the infrared beam penetrates (of the order 0.3-3 pm, dependent on its wavelength) just beyond the ATR crystal-specimen boundary before it is reflected back and makes its way through the crystal to the detector. On this short path (of the evanescent wave) into the sample surface layer, light is absorbed, and the reflected beam carries characteristic spectral information of the sample. The decaying amplitude of the evanescent wave and the depth of penetration dp at which it has decreased to a proportion of 1 /e is defined by the Harrick equation (Equation (2)), where X is the wavelength of the incoming... 

The major advantage of TIRF is that fluorophores outside the evanescent wave (typically more than 200 nm away from the surface) are not excited. Hence, TIRF has an intrinsic sectioning capability. Of interest is that the section capability (z-resolution) is far better than for confocal microscopy systems, which typically have a z-resolution of about 1 /mi. In addition and in contrast to confocal microscopy, TIRF does not cause out-of-focus bleaching because only the molecules at the surface will sense the evanescent wave. However, in comparison with confocal microscopy, a clear limitation of TIRF is that only one z-plane can be imaged the molecules immediately adjacent to the surface. As a consequence,... 

Since TIRF produces an evanescent wave of typically 80 nm depth and several tens of microns width, detection of TIRF-induced fluorescence requires a camera-based (imaging) detector. Hence, implementing TIRF on scanning FLIM systems or multiphoton FLIM systems is generally not possible. To combine it with FLIM, a nanosecond-gated or high-frequency-modulated imaging detector is required in addition to a pulsed or modulated laser source. In this chapter, the implementation with of TIRF into a frequency-domain wide-field FLIM system is described. 

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