Evanescent wave penetration length

Spectral information is gathered from species within the range of the evanescent wave penetration length at the ITO-solution interface. The reported cell design... 

Delta is of the order of 10 and is typically of the order of a few milliradians. As long as the beam is incident below this critical angle, it is totally reflected and only an evanescent wave penetrates the substrate. This has two very important consequences. First, the penetration depth is of the order of 20 A and thus one can signiflcantly discriminate in favor of a surface-contained material. Compton and elastic scattering are also minimized. In addition, the reflection enhances the local intensity by as much as a factor of 4 as well as the effective path length. All of these factors combined enhance the surface sensitivty of the technique and when combined with solid-state fluorescence detection, submonolayer amounts of material can be detected. ... 

Problem 7.3. The fluorescence spectrum will be sensitive to the population of the scattered atoms if the evanescent wave penetration depth, S, is less than, or comparable to, the population memory length Ip (see Section 2.4.3). This condition is fulfilled at incidence angles above 0, satisfying the equation... 

Problem 7.4. The vapor spectra excited by an evanescent wave will depend strongly on the change of the vapor polarization in vapor-surface scattering if the evanescent wave penetration depth is less than or comparable with the polarization memory length, i.e.,... 

The use of infrared spectroscopy in the Earth and environmental sciences has been widespread for decades however, until development of the attenuated total reflectance (ATR) technique, the primary use was ex situ material characterization (Chen and Gardella, 1998 Tejedor-Tejedor et al., 1998 Degenhardt and McQuillan, 1999 Peak et al., 1999 Wijnja and Schulthess, 1999 Aral and Sparks, 2001 Kirwan et al., 2003). For the study of environmental systems, the strength of the ATR-Fourier transform infrared (FTIR) technique lies in its intrinsic surface sensitivity. Spectra are collected only from absorptions of an evanescent wave with a maximum penetration depth of several micrometers from the internal reflection element into the solution phase (Harrick, 1967). This short optical path length allows one to overcome any absorption due to an aqueous phase associated with the sample while maintaining a high sensitivity to species at the mineral-water interface (McQuillan, 2001). Therefore, ATR—FTIR represents a technique capable of performing in situ spectroscopic studies in real time. 

Figure 7. Top to bottom Fresnel reflectivity, penetration length, evanescent wave intensity and reflected beam plane calculations for a model system. The parameter bu = 2/uk i Q], where is the scattering wavevector, Qc is the critical angle wavevector, and p is the linear absorption coefficient. [From Elements of Modem X-ray Physics by Als-Neilson and DesMorrow, with permission from the editors at John Wiley and Sons.]...

Figure 2. Three common configurations for spectroscopic monitoring of solution components generated at an electrode. With an optically transparent electrode, a beam passes through the electrode and absorbance U governed by an effective path length of approximately y l)t. The wavy line indicates an imaginary boundary layer of electrogenerated material (P) moving out into the solution. In the case of internal reflection, absorbance is determined by penetration of the evanescent wave into the solution. R indicates a reactant initially present in solution P is an electrogenerated product.

This field is known as an evanescent wave and has a decay length or penetration depth on the order of the wavelength of the incident light. 

Fig. 2. Fluorescence intensity ratio R (see text for details) as a function of the inverse of the penetration length A for the optical evanescent wave. The solid curve represents the best fit to a first-order polynomial expansion in powers of A". Polymer molecular weight is 120,000.

Excitation of the gas by an evanescent wave introduces a pure imaginary z-component of the wave vector kf and, hence, an additional time-of-flight broadening of the fluorescence lines. The fluorescence line intensity is determined by the gas volume where the polarization corresponding to the contributions given by either Eq. (7.40) or Eq. (7.41) is essentially nonzero. Therefore, the intensity of emission at the frequency w is proportional to the EW penetration depth, 5, whereas that at the frequency mo is proportional to the polarization memory length, lx = ut/t (see Section 2.4.3). The latter line is thus dominant in the spectrum if <5 [Pg.189]

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