Evanescent wave dynamic

Fig. 7. Schematic diagram of the evanescent wave dynamic light-scattering instrumentation (Reproduced with permission from Fytas et al., Science 274 2041 Copyright (1996) American Association for the Advancement of Science)...

Evanescent wave dynamic light scattering (EWDLS) can be used to study the rapid motion of submicron particles near and on interfaces. Two situations have been studied restricted diffusion of spherical particles moving near an interface, and composition fluctuations of polymers attached to the interface. 

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... 

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. 

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],... 

The fabrication and characterization of a fiber optic pH sensor based on evanescent wave absorption was presented by Lee63. The unclad portion of a multi-mode optical fibre was coated with the sol-gel doped with pH sensitive dye. The sensitivity of the device increased when the multiple sol-gel coatings were used in the sensing region. The dynamic range and the temporal response of the sensor were investigated for two different dyes -bromocresol purple and bromocresol green. 

The possibility of reflection of electrons by an evanescent wave formed upon the total internal reflection of femtosecond light pulses from a dielectric-vacuum interface is quite realistic. The duration of the reflected electron pulses may be as long as 100 fs. In the case of electrons reflecting from a curved evanescent wave, one can simultaneously control the duration of the reflected electron pulse and affect its focusing (Fig. lc). Of course, one can imagine many other schemes for controlling the motion of electrons, as is now the case with resonant laser radiation of moderate intensity [9, 10]. In other words, one can think of the possibility of developing femtosecond laser-induced electron optics. Such ultrashort electron pulses may possibly find application in studies into the molecular dynamics of chemical reactions [1,2]. 

FIG. 1b A magnified view of the spot where the laser beam intercepts the interface in Fig. la. The evanescent wave propagates in the y direction with an amplitude that is attenuated in the /. direction. A tethered polymer chain scatters the evanescent wave. From the properties of the scattered light it is possible to obtain a measure of the spatial distribution of the polymer material at the interface as well as a measure of the dynamical properties of the polymer chains. (From Ref. 9.)... 

The use of an evanescent wave to excite fluorophores selectively near a solid-fluid interface is the basis of the technique total internal reflection fluorescence (TIRF). It can be used to study theadsorption kinetics of fluorophores onto a solid surface, and for the determination of orientational order and dynamics in adsorption layers and Langmuir-Blodgett films. TIRF microscopy (TIRFM) may be combined with FRAP ind FCS measurements to yield information about surface diffusion rates and the formation of surface aggregates. 

Bordo, V., Loerke, J., and Rubahn, H.-G. (2001). Two-photon evanescent wave spectroscopy A new account to gas-solid dynamics in the boundary layer. Phys. Rev. Lett., 86 1490 -1493. 

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