Surface total reflection excitation

The single prism measurements can also be done using the configuration shown in Fig. 9. Here the thin metal film itself on the prism base serves as the low index gap for exciting SEW on the metal-air interface below. This type of attenuated total reflection excitation of surface waves was first used by... 

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

The background problem can be further overcome when using a surface-confined fluorescence excitation and detection scheme at a certain angle of incident light, total internal reflection (TIR) occurs at the interface of a dense (e.g. quartz) and less dense (e.g. water) medium. An evanescent wave is generated which penetrates into the less dense medium and decays exponentially. Optical detection of the binding event is restricted to the penetration depth of the evanescent field and thus to the surface-bound molecules. Fluorescence from unbound molecules in the bulk solution is not detected. In contrast to standard fluorescence scanners, which detect the fluorescence after hybridization, evanescent wave technology allows the measurement of real-time kinetics (www.zeptosens.com, www.affinity-sensors.com). 

Total reflection x-ray fluorescence (TXRF) has become very popular for the conduct of microanalysis and trace elemental analysis [77-79]. TXRF relies on scatter properties near and below the Bragg angle to reduce background interference, and to improve limits of detection that can amount to an order of magnitude or moreover more traditional XRF measurements. As illustrated in Fig. 7.18, if x-rays are directed at a smooth surface at a very small angle, virtually all of the radiation will be reflected at an equally small angle. However, a few x-rays will excite atoms immediately at the surface, and those atoms will emit their characteristic radiation in all directions. One obtains very clean... 

One can distinguish between methods in which absorption of the evanescent surface wave in different wavelength regions is measured (these are often called attenuated total reflection methods), and methods which use the evanescent wave to excite other, spectroscopic phenomena, like fluorescence and Raman scattering or light scattering. As the methods of conventional fluorescence spectroscopy have been shown to be exceptionally successful in studies of proteins and other biopolymers, their evanescent surface-sensitive counterparts will be reviewed first. 

By adjusting e (within the restriction e > 6r), Otto realized he could match k-,x of the surface wave [Eq. (3)] to the k, of the totally internally reflected wave [Eq. (8)] and hence excite the surface wave. Note the gap of thickness t shown in Fig. 8 is necessary to obtain the total internal reflection and that the surface wave is excited by the exponentially decaying fields below the prism. Ottoll found that when e satisfied... 

Otto, A. (1968). Excitation of nonradiative surface plasma waves in silver by the method of fimstrated total reflection. Zeitschrift fur Physik 216 398-410. 

Thus, surface plasmons (SPs) are excited on the dielectric medium - metal interfoce through evanescent wave excitation by attenuated total reflection (ATR) (23, 33, 36-38). One can excite SPs through the Kretschmann-Raether configuration... 

Otto, A. (1968) Excitation of Nonradiative Surface Plasma Waves in Silver by Method of Frustrated Total Reflection. Zeitschrift fur Physikalische 216 398-410. 

In ATR-FTIR excitation occurs only in the immediate vicinity of the surface ol the reflection element, in an evanescent wave resulting from total internal reflection. The intensity of the evanescent field decays exponentially in the direction normal to the interface with a penetration depth given by (1.7.10.121, which for IR radiation is of the order of a few hundreds of nm. Absorption leads to an attenuation of the totally reflected beam. The ATR spectrum is similar to the IR transmission spectrum. Only for films with a thickness comparable to, or larger than, the penetration depth of the evanescent field, do the band intensities depend on the film thickness. Information on the orientation of defined structural units can be obtained by measuring the dichroic ratio defined as R = A IA, where A and A are the band absorbances for radiation polarized parallel and perpendicular with respect to the plane of incidence, respectively. From this ratio the second-order parameter of the orientation distribution (eq. [3.7.13]) can be derived ). ATR-FTIR has been extensively used to study the conformation and ordering in LB monolayers, bilayers and multilayers of fatty acids and lipids. Examples of various studies can be found... 

In the experimental study of surface excitons various optical methods have been used successfully, including the methods of linear and nonlinear spectroscopy of surface polaritons. A particularly large body of information has been obtained by the method of attenuated total reflection of light (ATR), introduced by Otto (1 2) (Fig. 12.1) to study surface plasmons in metals. Later the useful modification of ATR method also was introduced by Kretschmann (3) (the so-called Kretschmann configuration, see Fig. 12.2). The different modification of ATR method has opened the way to an important development in the optical studies of surface waves and later was used by numerous authors for investigations of various surface excitations. 

The most common approach to excitation of surface plasmons is by means of a prism coupler and the attenuated total reflection method (ATR). There are two configurations of the ATR method - Kretschmann geometry [5] and... 

The attenuated total reflection method can be also used to excite coupled surface plasmons on thin metal films. The couphng of a hght into a symmetric or antisymmetric surface plasmon supported by a thin film (Sect. 2.2) can be in principle achieved in a geometry similar to the Otto geometry (Fig. 23) in which the semi-infinite metal is replaced by a thin metal film [20]. 

The potential of surface plasmons for optical sensing was recognized in the early 1980s when surface plasmons, excited in the Kretschmann geometry of the attenuated total reflection method, were used to probe processes at the surfaces of metals [1] and to detect gases [2]. Since then, numerous surface plasmon resonance (SPR) sensors have been reported. 

In TXRF, involving irradiation of an optically flat sample with a parallel X-ray beam below the angle of total reflection, the depth penetration of the primary X-rays is confined to a few tens of nanometers below the surface. The technique of a-XRF, based on the confinement of the analytical region of the sample, involves the localized excitation and analysis of a microscopically small area of the surface, providing information on the lateral distribution... 

Figure 4. Two configurations for evanescent wave optics, (a) Top total internal reflection of a plane wave at the base of a glass prism. Bottom the reflectivity R recorded by a detector as a function of the angle of incidence shows the increase to unity at 6, the critical angle for total reflection, (b) ATR setup for the excitation of surface plasmons (PSPs) in Kretschmann geometry. Top a thin metal film (thickness 50 nm) is evaporated onto the base of the prism and acts as resonator driven by the photon field. Bottom the resonant excitation of the PSP wave is seen in the reflectivity curve as a sharp dip at coupling angle 6g.

Spectra. A Perkin-Elmer Hitachi model 200 spectrophotometer was ised to record all ultraviolet spectra. The infrared spectra of the films were obtained using a Nicolet FTIR-T199. The transmission spectra of films were obtained from samples moistened in tetrachloroethylene (2) and mounted between NaCl plates. Attennuated total reflection (ATR) spectra were taken by placing the exposed side of the film in contact with a germanium crystal at a angle of incidence. Fluorescence from film surfaces were measured using a Perkin-Elmer MPF-i iiB fluorescence spectrometer. The excitation beam (3 0 nm, slit, i+nm) was incident on the film at and the emission (iiOO-500 nm) was measured at 90 to the excitation beam. 

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