Reflectivity interface

Attenuated Total Reflection (ATR).4c A sample brought in contact with the totally reflecting surface of a high-refractive-index material (the ATR crystal), will, on IR irradiation, give an evanescent wave in the less dense medium that extends beyond the reflecting interface. This wave will be attenuated in regions of the IR spectrum where the sample absorbs energy. Observation of such waves constitute ATR measurements. Only the small amounts of beads necessary to cover the area of the ATR crystal are required. 

Internal reflection spectroscopy (2), also known as attenuated total reflectance (ATR), is a versatile, nondestructive technique for obtaining the IR spectrum of the surface of a material or the spectrum of materials either too thick or too strongly absorbing to be analyzed by standard transmission spectroscopy. The technique goes back to Newton who, in studies of the total reflection light at the interface between two media of different retractive indices, discovered that an evanescent wave in the less dense medium extends beyond the reflecting interface. Infrared spectra can conveniently be obtained by measuring the interaction of the evanescent wave with the external less dense medium. 

If sample material is in contact with the totally reflecting surface of the prism, an evanescent wave in the sample extends beyond the reflecting interface and the evanescent wave will be attenuated in infrared regions.The intensity of this wave decays exponentially with the distance from the surface of the ATR crystal. Due to the fact that the electromagnetic field passes only a few micrometers of the sample, this method is insensitive to sample thickness and therefore useful for analysis of strong absorbing or thick materials. Influencing factors for FT-ATR-IR-spectroscopy are as follows ... 

Figure 1. Left Schematic of the coordinate system at a totally reflecting interface separating tux) media of refractive index n and n2. Right Standing wave pattern and exponential decay of the electric field vector into the less dense medium, 2.

Figure 2. Exponential decay in (E )2 with distance zfrom the reflecting interface.

The crystal truncation rod (CTR). We now calculate the scattering intensity for a semi-infinite lattice, i.e., which has only one reflecting interface. This sum is nearly... 

Experimental Apparatus. The TIRIF apparatus used in these experiments has been described in detail elsewhere (28). The incident light totally internally reflects at the quartz-aqueous Interface and produces a standing wave normal to the reflecting interface inside the quartz (the optically more dense medium) due to the superposition of the incident and reflected waves. The electric field of the standing wave has a non-zero amplitude (E ) at the interface which decays exponentially with distance (z) normal to the interface into the aqueous phase (the optically less dense medium), thereby creating a surface evanescent wave that selectively excites molecules within a few thousand angstroms from the surface. 

For thin samples, the second boundary condition may be modified to include either transmissive or reflective interfaces, as discussed in Section 2.1.3.1. 

If the internally reflecting interface is coated with a conducting material, such as a thin metal film, the p-polarized component of the evanescent wave may penetrate the metallic layer and excite surface plasmon waves. If the metal is nonmagnetic, such as a gold film, the surface plasmon wave is also p-polarized, which creates an enhanced evanescent wave. Because of the penetration of the electric field into the lower-refractive-index medium, the interaction is quite sensitive to the refractive index at the metal film surface. When the angle is suitable for surface plasmon resonance, a sharp decrease in the reflected intensity is observed, as can be seen in Figure 21-14. The resonance condition can be related to the refractive index of the metal film and can be used to measure this quantity and other properties of the surface. 

As concluded from Equation [3], perpendicular polarized incident light undergoes a phase shift of 180° upon reflection, i.e. there is a node at the reflecting interface resulting in zero electric field strength at this point. On the other hand, parallel polarized components remain in-phase. However, this conclusion holds no longer in the case of absorbing media. 

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