Optical techniques absorption

Recoilless Optical Absorption in Alkali Halides. Recently Fitchen et al (JO) have observed zero phonon transitions of color centers in the alkali halides using optical absorption techniques. They have measured the temperature dependence of the intensity of the zero phonon line, and from this have determined the characteristic temperatures for the process. In contrast to the Mossbauer results, they have found characteristic temperatures not too different from the alkali halide Debye temperatures. 

Rosen, H. Hansen, A.D.A. Dod, R.L. Novakov, T., "Application of of the Optical Absorption Technique to the Characterization of the Carbonaceous Component of Ambient and Source Particulate Samples " Lawrence Berkeley Laboratory Berkeley, California, 1977. 

Future developments will be focused on oxygen measurement in the high temperature reaction of the cycle using if possible optical absorption technique. 

In Ref.212 an optical absorption technique was developed in order to follow chemical diffusion in Fe-doped SrTi03, which enables a spatially resolved in-situ detection, i.e., makes it possible to follow the stoichiometry as a function of space and time at high temperatures. 

Reflection-Absorption Infrared Spectroscopy (RAIS) operating at the grazing-angle which is one of the most powerful resonance absorption phenomenon and is the most sensitive optical absorption technique available for measuring low concentrations of chemical componnds adhered to surfaces and even allows for the non-destractive mono layer chemical analysis on the surface. Figure 6 shows the FOC-GAP-FTIR spectra 5 and 10 ug/cm 2-CEES deposited as a trace residue on stainless steel surface. It is clear from the spectra shown that low limits of detection (LOD) achievable could be as low as nanograms/cm (milligrams/m ). 

In the polluted atmosphere, the ultra-violet optical absorption technique (UV-photometer) does not always provides quality ozone data. A possible influence of various interferences on the measuring procedure can be responsible for this. The chemiluminescence method of O3 detection is considerably fi eer from interferences. ... 

Using optical absorption techniques, Phelps and Molnar also verified (51) apd were able to deduce from their data a specific reaction rate at 300°K of 2 X 10 cm sec A theoretical calculation of this specific reaction rate (or cross section) has been carried out by Von Roos, but his result is smaller than the experimental value by a factor of 10". ... 

From the oxygen-containing compounds much attention was paid to the radiolysis of alcohols. Their relative permittivities are between those of the hydrocarbons ( 2) and water ( 80). The behavior of alcohols in radiation chemistry is in many respects similar to that of water. The electrons ejected from alcohol molecules can be trapped in solid alcohol matrices and can be observed by optical absorption techniques. In Uquids, pulse radiolysis measurements confirmed the presence of solvated electrons, for instance in methanol = 630 nm, Smax = 1,700 m moP, G(esoiv ) 0.5 pmol in the picosecond time range. On the other hand, as main product of decomposition, H2 gas forms with a yield comparable to the yield observed in liquid -alkane radiolysis (G(H2) 0.5 pmol J ) (Freeman 1970, 1974 Spinks and Woods 1990). 

Computed optical properties tend not to be extremely accurate for polymers. The optical absorption spectra (UV/VIS) must be computed from semiempiri-cal or ah initio calculations. Vibrational spectra (IR) can be computed with some molecular mechanics or orbital-based methods. The refractive index is most often calculated from a group additivity technique, with a correction for density. 

The changes in the optical absorption spectra of conducting polymers can be monitored using optoelectrochemical techniques. The optical spectmm of a thin polymer film, mounted on a transparent electrode, such as indium tin oxide (ITO) coated glass, is recorded. The cell is fitted with a counter and reference electrode so that the potential at the polymer-coated electrode can be controlled electrochemically. The absorption spectmm is recorded as a function of electrode potential, and the evolution of the polymer s band stmcture can be observed as it changes from insulating to conducting (11). 

As NRA is sensitive only to the nuclei present in the sample, it does not provide information on chemical bonding or microscopic structure. Hence, it is often used in conjunction with other techniques that do provide such information, such as ESCA, optical absorption. Auger, or electron microscopy. As NRA is used to detect mainly light nuclei, it complements another accelerator-based ion-beam technique, Rutherford backscattering (RBS), which is more sensitive to heavy nuclei than to light nuclei. 

Detectors based on established optical absorption and emission techniques,... 

Measurements of tropospheric HO and H02 concentrations have been accomplished by both direct and indirect means. Direct techniques are based on the measurement of the hydroxyl or hydroperoxyl radical using some physical property of the radicals themselves e.g. optical absorption. Indirect techniques refer to methods based on the measurement of compounds that are uniquely and/or quantitatively formed from or destroyed by HO or HO2. Examples of these techniques for both [HO ] and [HO2 ] will be given. 

Asmus et al. unambiguously identified a variety of [R2S.. SR2] radical cations in solution and measured their optical absorption spectra using pulse radiolysis techniques [133]. They proposed that the spectrum of [H2S. .SH2] arises from the transition in the three-electron S.. S... 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

Comparing the two optical transduction techniques (absorption or SPR) used in this work, we can conclude that SPR technique appears to be more suitable for gas sensing even if it presents some limitation regarding the suitable film thickness for SPR excitation. Moreover, the response and recovery times during the anal5fle/sensing layer interaction appears shortest in the case of optical absorption measurements. Further investigations are in... 

Fluorescent chemical sensors occupy nowadays a prominent place among the optical devices due to its superb sensitivity (just a single photon sometimes suffices for quantifying luminescence compared to detecting the intensity difference between two beams of light in absorption techniques), combined with the required selectivity that photo- or chemi-luminescence impart to the electronic excitation. This is due to the fact that the excitation and emission wavelengths can be selected from those of the absorption and luminescence bands of the luminophore molecule in addition, the emission kinetics and anisotropy features of the latter add specificity to luminescent measurements8 10. 

SHG, more so than other optical techniques, can give deceptive results due to propagation phenomena. Disregarding for the moment optical absorption effects, non-phase-matched SHG displays the behavior pictured in Figure 2 (18). The harmonic intensity I w emerging from a sample depends on the sample thickness / in a... 

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. Then, the intermediates can be detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR study of radicals in the solid state is done on single crystals, polycrystalline samples or frozen aqueous solution. In case of polycrystalline samples or frozen aqueous solution the identification of the radicals from the ESR spectra is difficult in many cases and, for better identification, the ESR experiment should be conducted on irradiated single crystals. Later, the method of spin trapping, developed for the liquid phase5, was extended to polycrystalline solids. In this technique the polycrystalline solids are /-irradiated and subsequently dissolved in a solution containing the spin trap. 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

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