Modulated reflectance techniques

In contrast to the techniques that we have considered in the previous section, which have involved the measurement of an electrochemical quan- 

This may be understood more easily from Fig. 109(a) [185]. Here, we imagine a perturbation that alters the position of the bands with respect to each other vertically on the E vs. k diagram (i.e. in the Brillouin zone). Clearly, such a perturbation will alter the energy of the optical absorption onset and large differential signal will be seen at threshold. The effect is to 

The basis of electroreflectance is more subtle than thermoreflectance Fig. 109(b) shows that the main effect arises from the fact that the presence of an electric field destroys the translational symmetry along one of the directions of the crystal. This loss of symmetry means that k need no longer be conserved along that axis and optical transitions need no longer be vertical in the Elk diagram. As for thermoreflectance, this effect will be most marked at the critical points, again allowing the spectroscopist to extract the data of real interest from the otherwise rather shapeless absorption spectrum of the solid. 

The early treatments of the electroreflectance effect concentrated on the case of uniform electric fields and zero thermal broadening and was therefore suitable only for very lightly doped samples at very low temperatures. The optical properties of a solid are contained in the dielectric function. This function is complex, with the imaginary part only non-zero if the material actually absorbs light. The imaginary part of the dielectric function, e2, can be written for a single band-to-band transition as [186] 

The relationship between eqns. (637) and (638) may be seen if we recall that in the limit 2 - 0 

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A schematic of a PL system layout is shown in Figure 5. This optical system is very similar to that required for absorption, reflectance, modulated reflectance, and Raman scattering measurements. Many custom systems are designed to perform several of these techniques, simultaneously or with only small modifications. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

Optical techniques - including ellipsometry, Raman spectroscopy, potential modulation reflectance, and photo-electrochemical technique - and an understanding of what really happens during the passivation process... 

Boujday S, Methivier C, Beccard B, Pradier C-M (2009) Innovative surface characterization techniques applied to immunosensor elaboration and test comparing the efficiency of Fourier transform-surface plasmon resonance, quartz crystal microbalance with dissipation measurements, and polarization modulation-reflection absorption infrared spectroscopy. Anal Biochem 387 194-201... 

The first in situ Infrared Reflectance Spectroscopy rmder electrochemical control of the working electrode in a three-electrode cell was realized by Beden et al. using the so-called Electrochemical-ly Modulated Infrared Reflectance Spectroscopy (EMIRS). Experimental details of this external reflection technique are fully described in text books. °... 

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... 

There is a continuous interest in the spectroscopic studies of adsorbed species (for instance, [EMIRS], potential modulated reflectance spectroscopy [PMRS]). The EQCM technique was used recently for the study of chemisorption and oxidation of methanol at polycrystalline (platinized) Pt. It was found that accumulation of strongly adsorbing species at the electrode surface is accompanied by a decrease in the mass, relative to the background electrolyte, and the rate of this decrease is an indication how the surface becomes covered by adsorbates. 

One of the most commonly applied IR techniques developed to overcome these problems is the external reflectance technique. In this method, the shong solvent absorption is minimized by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The sensitivity problem, that is, the enhancement of the signal/noise ratio in the case of external reflectance techniques is solved by various approaches. These are, for instance, electrochemically modulated infrared spectroscopy (EMIRS), in situ FTIR (which use potential modulation), and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS, FTIR) [86,117-123]. 

Specular reflection spectroscopy has been actively used in in situ studies of the formation and optical behaviour of monolayer films on surfaces, and for detecting intermediates and products of heterogeneous chemical and electrochemical reactions. The vibrational spectra of the adsorbed species at electrode surfaces are obtained by surface-enhanced Raman scattering and infrared reflectance spectroscopies. Since the mid-1960s, modulated reflection spectroscopy techniques have been employed in elucidating the optical properties and band structure of solids. In the semiconductor electroreflectance, the reflectance change at the semiconductor surface caused by the perturbation of the dielectric properties of... 

A number of methods have been used to measure the EO coefficients and ni of the poled polymer samples. These EO measurements are made by detecting the change in refractive index of the poled polymer sample when a modulating electric field is applied to the sample. Mach-Zehnder [42,43]. Fabry-Perot [44,45], and Mich-elson [46] interferometric techniques have been used to evaluate the EO coefficients. Other techniques, such as an attenuated total reflection technique [47,48] and an ellipsometric technique [49-51], have also been employed to determine the r coefficients. 

The application of interference techniques overcomes the limitations exerted by the large optical wavelengths. With commercial phase-measurement interference microscopes (PMIM), a surface resolution of the order of 0.6 nm can be achieved [33, 34]. In a microscope a laser beam is both reflected from the sample surface and from a semitransparent smooth reference surface (Fig. 3). The interference pattern is recorded on an area detector and modulated via the piezo-electric driven reference surface. The modulated interference pattern is fed into a computer to generate a two-dimensional phase map which is converted into a height level contour map of the sample surface. While the lateral resolution (typically of the... 

Electrochemical impedance spectroscopy leads to information on surface states and representative circuits of electrode/electrolyte interfaces. Here, the measurement technique involves potential modulation and the detection of phase shifts with respect to the generated current. The driving force in a microwave measurement is the microwave power, which is proportional to E2 (E = electrical microwave field). Therefore, for a microwave impedance measurement, the microwave power P has to be modulated to observe a phase shift with respect to the flux, the transmitted or reflected microwave power APIP. Phase-sensitive microwave conductivity (impedance) measurements, again provided that a reliable theory is available for combining them with an electrochemical impedance measurement, should lead to information on the kinetics of surface states and defects and the polarizability of surface states, and may lead to more reliable information on real representative circuits of electrodes. We suspect that representative electrical circuits for electrode/electrolyte interfaces may become directly determinable by combining phase-sensitive electrical and microwave conductivity measurements. However, up to now, in this early stage of development of microwave electrochemistry, only comparatively simple measurements can be evaluated. 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

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