Modulated reflection spectrum

Figure Al.3.17. Modulated reflectivity spectrum of silicon. The theoretical curve is obtained from an empirical pseudopotential calculation [25], The experimental curve is from a wavelength modulation experiment from [32],...

Figure 13.5 Potential modulated reflectance spectrum of p-aminonitrobenzene (PANB) on platinum (solution phase 0.5 mM Na2S04 + 0.05 mM PANB). Applied dc 0.44 V vs. SHE. Modulation amplitude 50 mV. Modulation frequency 33 Hz. Incidence angle 65°. 11 signifies incident polarization parallel to incident plane and perpendicular to electrode surface. J signifies incident polarization perpendicular to incident plane (hence parallel to electrode surface). [From Ref. 50.]...

Absolute Reflection Spectrum versus Modulated Reflection Spectrum... 

Specular reflection spectroscopy enables us to study the redox behaviour of adsorbed species at a mirror electrode when either or both oxidized and reduced forms of the adsorbed species exhibit strong absorption bands in the UV-Vis region. In this case, the mode of measurement is, in fact, the absorption spectra of the adsorbed species at the electrode surface [55]. The potential modulated reflectance spectrum of the adsorbed molecules in the vicinity of its formal potential is given by the following equation ... 

Electronic spectra (Table 1.1, Fig. 1.2) have been measnred for the orange soln-tions of (RuO ] in aqueous base from 250-600 nm. [212-215, 222], and reproduced [215, 222]. There are two at 460 and 385 nm. [212, 213, 222] or three bands in the visible-UV region, at 460, 385 and 317 nm [214, 215]. These appear to be at the same positions as those for [RuO ] but the intensities and hence the general outline of the two spectra are very different. Woodhead and Fletcher reviewed the published molar extinction coefficients and their optimum values / dm (mol" cm" ) are 1710 for the 460 nm. band, 831 for the 385 nm. band and 301 for the 317 nm. band - the latter band was not observed by some workers [214]. The distinctive electronic spectrum of ruthenate in solution is useful for distinguishing between it, [RuO ]" and RuO [212, 222]. Measurements of the electronic spectra of potassium ruthenate doped in K CrO and K SeO and of barium ruthenate doped into BaSO, BaCrO, and BaSeO (in all cases the anions of these host materials are tetrahedral) indicate that in that these environments at least the Ru is tetrahedrally coordinated. Based on this evidence it has been suggested that [RuO ] in aqueous solution is tetrahedral [RuO ] rather than franx-[Ru(0H)3(0)3] [533, 535]. Potential modulated reflectance spectroscopy (PMRS) was used to identify [RuO ] and [RuO ] " in alkaline aqueous solutions during anodic oxidation of Ru electrodeposited on platinum from [Ru3(N)Clg(H30)3] [228]. 

The possibility of 1.5 pm ultrafast all-optical modulator realization based on GaAs/(AlGa)jiOy heterostructures is discussed. The excitation of the samples by 150fs laser pulses leads to about 25 nm shift of the reflection spectrum. The mean decay time for nonlinear reflection in heterostmctures ranges from 1.0 to 2.5 ps. 

Fig. 16. Effect of the addition of 0.5% ( ), 1.0% (x), and 3.0% (O) H20 to 0.5 M D2S04 on the differential reflectance spectrum of a gold film, modulated around the potential of zero charge.

Fig. 23. Reflectance spectrum from Pt/1 M H2S04, 0.5 M methanol. Modulation from 0.05 to 0.45M (NHE) at 8.5Hz. Scan rate 0.0127pm s. ...

The basic principle of the PM-IRRAS method (Fig. 4.1) is to combine the FT-IRRAS experimental conditions with a fast modulation of the polarization state of the incident electric field (ideally between p and s hnear states) and to extract from the detected intensity (using electronic filtering and demodulation) the two signals (Rp-Rg) and (Rp-t-RJ in order finally to compute the differential reflectivity spectrum AR/R [18]. 

The modulation technique resolves the main technical difficulties of the constant potential spectral measurement at the electrode surface. Basically, in the modulation spectroscopic measurement, only the change associated with the change of the modulated parameter is detected. Thanks to lock-in ampHfication, we can significantly increase the sensitivity of the optical signal detection. It must be noted that what we can obtain is the change of the spectrum with respect to the modulated parameter but not the absolute reflection spectrum. In other words, modulation methods give the difference or differential spectrum. From the modulation spectrum, we cannot obtain explicitly the absolute spectrum at a unique condition unless a perfect reference absolute reflection spectrum is already in our hands. One should be careful in the interpretation of the spectral curves at this point. 

The polarization modulation reflection absorption spectrum acquired using the SSD can therefore be written as... 

Bewick and Tuxford " have used ac electromodulation reflectance spectroscopy with square wave modulation (stair case) from a potential in the double-layer range to various potentials in the anodic film range. They report the detection of film formation already at Eu = 0.7 V and that the signal associated with the film formation increases until 1.5 V where it levels off. They conclude that the general features of the wavelength dependence of the reflectance spectrum are determined primarily by the wavelength dependence of the optical properties of the Pt substrate and not by the film. 

The discussion above illustrates how the application of hydrostatic or uniaxial stress can shift the exciton energy and features such as peaks A and A in the reflection spectrum. If such pertu2d>ations aire applied periodically and the reflected light analysed using a phase sensitive detector than various spectral features %diich are not obvious in the direct reflection spectrum can be enhanced (30). The reflectance modulation is given by... 

The high-frequency modulation between s- and p-polarization allows a simultaneous measurement of the difference spectrum between s- and p-polarized incident light. The PM-IRRA method yields the polarization modulated reflectivity S, as defined by Equation 2.23 [71,74]. 

The sub-Doppler structure of the SR spectrum provides an opportunity to determine the frequency shift caused by the atom-surface interaction with high resolution. This singularity can be isolated if one applies a low-frequency modulation to the incident light. By detecting the amplitude-modulated reflected signal one obtains for the reflectivity at the modulation frequency, cOm,... 

The ordered, periodic structure results in periodic modulation of the dielectric constant perpendicular to the substrate surface, along the (111) direction of the film, which produces sharp, characteristic reflection peaks. Figure 3.88 shows the reflectance spectrum for 200 nm colloidal polystyrene particles at each of the stages from the preceding scheme. The reflection peak changes from 822 nm for the template assembly... 

The generalization to the case of a thermally averaged parent state describes an interesting modulation curve that reflects in position and width the rotational eigenvalue spectrum of the resonant intermediate [31]. This structure has been observed in studies of HI ionization in Ref. 33. A schematic cartoon depicting the excitation scheme and the form of the channel phase for the case of a thermally averaged initial state is shown in Fig. 5g. 

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