Polarization-modulation principles

RAIRS is a non-destructive infrared technique with special versatility - it does not require the vacuum conditions essential for electron spectroscopic methods and is, therefore, in principle, applicable to the study of growth processes [4.270]. By use of a polarization modulation technique surfaces in a gas phase can be investigated. Higher surface sensitivity is achieved by modulation of the polarization between s and p. This method can also be used to discriminate between anisotropic near-sur-face absorption and isotropic absorption in the gas phase [4.271]. 

For optimum performance, CD measurements require a polarization modulated source. In principle, any of the polarization-selective optical devices discussed earlier could be mechanically moved to create the required modulation. However, this approach is problematic in that it is difficult to implement physically, the mechanical movement may introduce noise into the measurement situation, and there are limitations to the rate at which the polarization can be modulated. A preferable approach is to use an electronic device to effect the required phase retardation. Although a number of devices have been used for this purpose (e.g. magneto-optical, Kerr effect, etc.), modern CD instruments rely upon either the Pockels effect, or photoelastic modulation for this function. 

A second lock-in detection method that was employed is polarization modulation, which involves modulating the polarization state of the incident infrared beam, and is again an extension of an approach developed for the study of the gas-solid interface [80]. Polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) relies upon the principles underlying the surface selection rule... 

PM-IRRAS exploits the different attenuation of s- and p-polarized light by adsorbed species at a reflective (electrode) surface to annul the unchanging contributions to the infrared signal at the detector from the solvent, window, and so on, and produces an absolute rather than difference spectrum at a particular potential. In this approach, a photo-elastic modulator is employed to modulate the polarization state of the incident infrared ray between s- and p-states. On the basis of Greenler s theory [81, 82], this polarization modulation gives rise to an AC signal at the detector, which is proportional (/p —7s)-the difference in intensity of the two polarizations. Since, in principle, /p is absorbed... 

VCD instruments, based also on the principles depicted in Figure 13, are in comparison to the ECD instruments, relatively new products on the market. Thus, there have been further developments. Two of them should be mentioned, namely, the use of the Fourier transform technology and the use of a second PEM with and without a second polarizing element. This dual polarization modulation (DPM) method increases the quality of the VCD spectra. Also, the Raman spectrometers for ROA, with which the difference of scattered left and right circularly polarized Raman light is measured, have been developed to a standard that allows systematic chiroptical analyses, nowadays. 

Under potentiostatic conditions, the photocurrent dynamics is not only determined by faradaic elements, but also by double layer relaxation. A simplified equivalent circuit for the liquid-liquid junction under illumination at a constant DC potential is shown in Fig. 18. The difference between this case and the one shown in Fig. 7 arises from the type of perturbation introduced to the interface. For impedance measurements, a modulated potential is superimposed on the DC polarization, which induces periodic responses in connection with the ET reaction as well as transfer of the supporting electrolyte. In principle, periodic light intensity perturbations at constant potential do not affect the transfer behavior of the supporting electrolyte, therefore this element does not contribute to the frequency-dependent photocurrent. As further clarified later, the photoinduced ET... 

Based on the results obtained in the investigation of the effects of modulation of the electron density by the nuclear vibrations, a lability principle in chemical kinetics and catalysis (electrocatalysis) has been formulated in Ref. 26. This principle is formulated as follows the greater the lability of the electron, transferable atoms or atomic groups with respect to the action of external fields, local vibrations, or fluctuations of the medium polarization, the higher, as a rule, is the transition probability, all other conditions being unchanged. Note that the concept lability is more general than... 

The measurement of vibrational optical activity requires the optimization of signal quality, since the experimental intensities are between three and six orders of magnitude smaller than the parent IR absorption or Raman scattering intensities. To date all successful measurements have employed the principles of modulation spectroscopy so as to overcome short-term instabilities and noise and thereby to measure VOA intensities accurately. In this approach, the polarization of the incident radiation is modulated between left and tight circular states and the difference intensity, averaged over many modulation cycles, is retained. In spite of this common basis, there are major differences in measurement technique and instrumentation between VCD and ROA consequently, the basic experimental methodology of these two techniques will be described separately. 

For the measurement of differential absorption spectra in the visible and ultraviolet spectral region, a light modulation technique was introduced over two decades ago [7]. The principle of this measurement also governs the measurement of infrared VCD. This technique involves the high frequency modulation of the exciting beam between left and right circular polarization states via a photoelastic modulator, and measuring the differential absorption with a lock-in amplifier tuned to the modulation frequency. 

An interesting approach is a method that uses the modulation of the degree of circular polarization (e.g., between pure 1-cpl and linearly polarized light) to determine fast isomerization kinetics [71]. However, preliminary results on [Co(ox)(phen)2] and rhodamin 6G 14 were not followed by the announced full papers, and the arcticle is rarely cited. In principle the concept may be used to determine interconversion and the CD spectra of rotamers (like 14) which are the reactants in asymmetric synthesis (see Sec. II.C.). 

Consider a plane-parallel condenser of capacitance C whose plates are a p-type semiconductor (e.g., a CP) and a metal, and polarize the latter negatively. Excess positive charges (i.e., holes) appear at the surface of the semiconductor, and since its conductivity is low, they are in fact distributed over a certain thickness within the material. These excess holes, or at least part of them, should take part in the conduction. Applying a voltage to an external electrode not in contact with the semiconductor modulates its conductivity. This is the principle of the field effect, and clearly this control of the current through a gate electrode opens the possibility of transistor action without requiring the existence of p-n junctions. 

Optical biosensors can be defined as sensor devices which make use of optical principles for the transduction of a biochemical interaction into a suitable output signal. The biomolecular interaction on the sensor surface modulates the light characteristics of the transducer (i.e., intensity, phase, polarization, etc.), and the biosensing event can be detected by the change in diverse optical properties such as absorption, fiuorescence, luminescence or refractive index, among others. 

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