Polarized specular-reflectance technique

Zapanta-Le Geros et al. (1970) have recently reviewed the subject of infrared spectra of carbonate-containing synthetic and biological apatites. Klein et al. (1970) have used the polarized infrared specular-reflectance technique to study single crystals of apatites. They found this technique to be a more powerful method for the analysis of the vibration spectra of crystalline structures than the powder absorption techniques. 

When the sample is deposited on the surface of a smooth mirror-like substrate, it is possible to use the specular reflection technique, or external reflection spectroscopy (ERS), which is carried out with the beam at near normal incidence [28]. The specular reflectance is completely governed by Fresnel s formalism and is predominately a function of refractive index. The impinging light reflects from the sample surface and does not penetrate the sample. If the surface is smooth, the reflection and the incidence angles are equal and the reflected beam retains the polarization characteristics of the impinging beam. This type of reflection is called regular Fresnel reflection [29]. 

A powerful characteristic of RAIR spectroscopy is that the technique can be used to determine the orientation of surface species. The reason for this is as follows. When parallel polarized infrared radiation is specularly reflected off of a substrate at a large angle of incidence, the incident and reflected waves combine to form a standing wave that has its electric field vector (E) perpendicular to the substrate surface. Since the intensity of an infrared absorption band is proportional to / ( M), where M is the transition moment , it can be seen that the intensity of a band is maximum when E and M are parallel (i.e., both perpendicular to the surface). / is a minimum when M is parallel to the surface (as stated above, E is always perpendicular to the surface in RAIR spectroscopy). 

Ellipsometry involves measurement of the change of polarization that occurs when polarized radiation is reflected from a specular surface. Ellipsometry is an extension of the reflection technique in which the polarization of the reflected radiation rather than just its intensity is measured. Ranges of operation from ultra-violet to infrared, UV/Visible is typical. 

Reflectance spectroscopy has the advantage that can be used both, in solid materials, and/or in concentrated dispersions. Although Uv-Vis reflection techniques have been used in some applications, " they have not been extensively tested for monitoring emulsion polymerization reactions. Yu et al used reflectance to monitor imidization reactions. Specular reflection spectra from polarized light combined with a Kramers-Kroning transformation were used to assess the surface orientation of PET sheets. NIR with monochromatic laser light has been used to monitor the particle size distribution. The main problem for the interpretation of... 

Reflectance spectroscopy is commonplace for samples that cannot be prepared for transmittance measurements. However, reflectance measurements must be carefully conducted as the reflected beam is not only indicative of the composition of the sample but is also affected by surface conditions at the sample plane. This makes the reflectance spectra, though indicative of material chemistry, difficult to interpret and generally less useful for quantitative analysis. Since the polarization of the beam is maintained for reflectance, especially specular reflectance methods, examination of orientation at polymer surfaces using reflection techniques is attractive [10]. Reflection-absorption modes involve the transmission of the infrared beam through the sample and subsequent reflection to pass through the sample again. Usually, sample preparation is difficult for such experiments and they... 

Investigation of the mechanical, adhesion, and electrical behavior of ultrathin polymer films requires spatial resolution of the order of 10-100 °A. Specular reflectance spectroscopy is a most convenient technique for probing structures on either metallic or dielectric surfaces. Furthermore, techniques such as polarization modulation spectroscopy have significantly increased sensitivity and the possibility of photoacoustic spectroscopy is also available. 

Ellipsometry was in fact the earliest optical technique to be applied to the study of electrode processes. It involves the determination of the change in polarisation state of an obliquely incident light beam upon specular reflection at a surface. In order to fully define a monochromatic light beam it is necessary, in addition to knowing the frequency, amplitude, and direction of propagation, to include information about the electric and magnetic vectors which describe the polarisation state. Since these vectors are orthogonal and related in amplitude it is, in fact, only necessary to consider one of them, and it is the electric vector that is usually chosen. If this vector lies in a plane then the beam is said to be plane polarized, and further if this plane lies parallel to the plane of incidence at a surface the beam is said to be p-polarised, whereas a beam polarised in a plane perpendicular to the plane of incidence is referred to as s-polarised. For any beam it is possible to resolve the electric vector into its s and p components, and when these components are of the same frequency but different phase and amplitude the beam is said to be elliptically polarised. This name arises from the... 

There are surface-oriented sampling techniques such as internal reflectance (ATR) and external (specular) reflectance, both providing data that are influenced in one way or another by the sample s refractive index (at the measurement wavelength), the refractive index of the sampling medium (in particular, ATR), and the polarization of the infrared beam (in particular, external reflectance). For ideal cases, the mathematical relationship between the recorded spectrum and an idealized absorption (or transmission) spectrum is understood. In these situations, a correction algorithm can be... 

The last technique to be described here is ellipsometry. This has been used for many years by physicists to study adsorbed films on a large variety of surfaces. Recently it gained some popularity with surface chemists, especially surfaces. Recently it gained some popularity with surface chemists. The experimental set-up is somewhat similar to that used for specular reflection spectroscopy but differs from it in one important aspect which is a result of the parameters measured in ellipsometry. A schematic diagram of the optical part of an ellipsometer is shown in Fig. 81. Plane polarized light of a definite wavelength (this is the crucial difference) shines on the electrode surface and... 

IRE Techniques IRE involves taking measurements of the state of polarization of reflected light. As illustrated in Eigure 2.17, linearly polarized light is specularly... 

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