Specular reflection technique

The technique developed by Bewick for use with a dispersive IR spectrometer is termed electrically modulated infrared spectroscopy (EMIRS) and is essentially a direct development of the UV-VIS specular reflectance technique [50], modulated specular reflectance spectroscopy (MSRS). As in MSRS, radiation is specularly reflected form a polished electrode surface while the electrode potential is modulated with a square wave between a base potential and the working potential at which the process of interest occurs the wavelength range of interest is then slowly scanned. Only that... 

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. 

Specular reflectance techniques basically involve a mirror-like reflection from the sample surface that occurs when the reflection angle equals the angle of incident radiation. It is used for samples that are reflective (smooth surface) or attached to a reflective backing. Thus, specular techniques provide a reflectance measurement for reflective materials, and a reflection-absorption (transflectance) measurement for the surface films deposited on, or pressed against reflective surfaces (Figure 9). 

The specular-reflectance technique is frequently used to measure layer thickness, especially in the semiconductor industry. The system can be calibrated with known-thickness samples by relating band intensities to these thicknesses and graphing the results. The graph can be used to determine the layer thickness of the samples after their band intensities have been determined. Whenevei the composition of the coating or the type of substrate is changed, the system must be recalibrated. 

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]. 

In ERS, the beam makes a high-angle reflection of approximately 88° from the sample. Single or multiple reflections can be used. For the analysis of coatings, multiple reflections are generally required. The ERS method has been particularly valuable for examining coatings and adhesives on the surfaces of metals [30]. The problem with the specular reflection technique is that because of the optical requirement of a mirror-like optical substrate, few practical samples can be studied. 

Though a powerfiil technique, Neutron Reflectivity has a number of drawbacks. Two are experimental the necessity to go to a neutron source and, because of the extreme grazing angles, a requirement that the sample be optically flat over at least a 5-cm diameter. Two drawbacks are concerned with data interpretation the reflec-tivity-versus-angle data does not directly give a a depth profile this must be obtained by calculation for an assumed model where layer thickness and interface width are parameters (cf., XRF and VASE determination of film thicknesses. Chapters 6 and 7). The second problem is that roughness at an interface produces the same effect on specular reflection as true interdiffiision. 

Infrared spectroscopy, including Fourier-transform infrared (FTIR) spectroscopy, is one of the oldest techniques used for surface analysis. ATR has been used for many years to probe the surface composition of polymers that have been surface-modified by an etching process or by deposition of a film. RAIR has been widely used to characterize thin films on the surfaces of specular reflecting substrates. FTIR has numerous characteristics that make it an appropriate technique for... 

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). 

The three principal X-ray techniques that have been applied to the study of structure at the electrode/electrolyte interface are diffraction, absorption and specular reflection. 

Whereas the XSW technique takes advantage of the standing wave established on the total reflection of X-rays from a mirror surface, a conceptually more straightforward approach is that of simply specularly reflecting an X-ray beam from an electrode coated with the film of interest, measuring the ratio of the intensities of the incident and reflected rays, and fitting the data, using the Fresnel equations, to a suitable model an approach similar to optical ellipsometry. 

Specular reflectance infrared involves a mirrorlike reflection producing reflection measurements of a reflective material or a reflection-absorption spectrum of a film on a reflective surface. This technique is used to look at thin (from nanometers to micrometers thick) films. 

Within the IR spectroscopy arena, the most frequently used techniques are transmission-absorption, diffuse reflectance, ATR, specular reflectance, and photoacoustic spectroscopy. A typical in situ IR system is shown in Fig. 7. Choosing appropriate probe molecules is important because it will influence the obtained characteristics of the probed solid and the observed structure-activity relationship. Thus, the probe molecules cover a range from the very common to the very rare, in order to elucidate the effect of different surfaces to very specific compounds e.g. heavy water and deuter-ated acetonitrile, CDsCN). The design of the IR cell is extremely important and chosen to suit the purposes of each particular study. For catalytic reactions, the exposure of catalytic metals must be eliminated in cell construction, otherwise the observed effect of the catalyst may not be accurate. 

Fig. 6.97. Some techniques used in the study of isotherms (a) Electrochemical quartz crystal microbalance mass change (AW) vs. quantity of electricity (AG) [Au, 0.1 M HCI04 (a), and 0.05 MH2S04 ( )] and anion coverage vs. electrode potential [poly-Au (open symbols), and Au(111) (dark symbols) HS04 (circles) and CI04 (triangles)]. Reprinted from H. Uchida, N. Ideda, and M. Watanabe, J. Electroanal. Chem. 42 , copyright 1997, Figs. 3 and 5, with permission of Elsevier Science.) (b) Specular reflection method reflectivity change vs. potential [Au, HCI04 with Nal (b) and (c) 0...

With all its complications and uncertainties, impedance spectroscopy, as seen at the end of the twentieth century, is a growing technique in fundamental electrodic analysis [cf. the seminal contributions of (independently) D. D. and J. R. MacDonald]. Among its advantages is that the necessary equipment is less expensive than that of competing spectroscopic equipment and that it can provide information on any electrochemical situation (e.g., it is not limited by, say, the need for specular reflectance, as in ellipsometry). 

Many techniques are based on this principle and can be used for the analysis of all types of samples. The spectrum obtained from reflected light is not identical to that obtained by transmittance. The spectral composition of the reflected beam depends on the variation of the refractive index of the compound with wavelength. This can lead to specular reflection, diffuse reflection or attenuated total reflection. Each device is designed to favour only one of the above. The recorded spectrum must be corrected using computer software. 

Figure 3.7 Spectroelectrochemical techniques (A) transmission, (B) internal reflectance, (C) specular reflectance, (D) parallel.

Spectroelectrochemical measurements can be made at conventional nontransparent electrodes by specular reflectance [23-25]. The optical beam is passed through the electrolyte and reflected from the electrode surface as shown in Figure 3.7C. This technique has been used effectively to study electrode mechanisms and to observe changes in the electrode surface itself. 

The techniques used in this stage of the work were all IR spectroscopy using a Perkin-Elmer spectrometer 599 and appropriate attachments. Conventional transmission through a sodium chloride cell of path length 0.1 mm, and multiple specular reflectance from aluminium films with a mirror finish were both used. 

To validate the whole technique, a transmission spectrum of the neat liquid silane was compared with one obtained by specular reflectance from an aluminium mirror surface on which a film of silane had been cast from 1% solution in anhydrous methanol. These two spectra were virtually indistinguishable, as regards both position and intensity of all peaks. They contained a considerable number of peaks, most of which could be assigned with complete satisfaction in terms of the known structure of y-glycidoxypropyltrimethoxy-silane. 

Specular and Diffuse Reflectance. Reflectance techniques have been applied primarily to samples which do not permit observation by transmission. Flat surfaces such as those of metal foils and single crystals can he studied by specular reflectance, whereas rough surfaces such as those of powders must be observed by diffuse reflectance. In both cases FT spectroscopy offers strong advantages in terms of the time required to acquire a spectrum. 

Unlike classical analytical spectroscopy performed on liquids or dilute solutions of analytes, diffuse reflectance measurement in the near-infrared must deal with a composite effect of spectroscopic absorption and scattering from the analyte and the matrix in which it is found. Differences in refractive indices of the sample material, specular reflection and observance of relatively small differences are all dealt with in this technique. 

---