Optical diffusion studies

The late 30 s brought a further important step in the investigation of the interstellar medium — the discovery of the first molecular species. In the optical region, the electronic spectra of the diatomic radicals CH, CH+, and CN, seen in absorption against the continuum spectra of bright background stars, furnished the first evidence that the interstellar medium was not devoid of molecules but contained at least some simple ones. However, the intensities of the molecular spectral peaks seen via optical absorption studies were quite weak compared with the spectra of atoms, indicating that the sources observed in these early studies were not rich in molecules. These sources, now labeled diffuse interstellar clouds, possess very low gas densities (n 102 cm-3) and are of limited interest chemically. 

Shaman, D. M. Vergo, N. (1988b) Optical (diffuse reflectance) and Mossbauer spectroscopic study of nontronite and related iron smectites. Amer. Mineral., 73, 1346—54. 

The maximum deviations from stoichiometry in CdTe have been found to be about 1017 excess Te or Cd atoms per cc., and in PbTe about 4 X 1018 excess Pb and 1019 excess Te atoms per cc. Because these deviations from a 1 to 1 stoichiometry are so small (5 X 10-4 or less), they cannot be studied by chemical analysis, x-ray diffraction, or density measurements. Rather the extent of nonstoichiometry and the nature of the defects must be inferred from such quantities as obtained from electrical, optical, magnetic, and self-diffusion studies. 

The fluorescent spectroscopy is one of the optical techniques that are widely used in the study of structure and dynamic properties of lipids in lamellar phases [76]. The Fluorescence Recovery after Photobleaching (FRAP) is successfully applied in the lateral diffusion studies of BLM (e.g. 77]. FRAP has been employed to study similar phenomena at the air/water interface of Langmuir trough [78]. 

Karger et al. [18,19] and later on developed by Schemmert [20] and Karger et al. [21] to a very sophisticated method, when employing optical interference microscopy (IFM). The design of the experiments using interference microscopy for diffusion studies (DIFM), the data acquisition through this type of experiment, and the evaluation of selected data obtained by DIFM are described in Sect. 3. 

The evidence from these methods suggests that lower values of diffusion constants are obtained from methods which are sensitive to factors important over longer distances, and so indicate that defects can have a controlling effect in diffusional rates. This is shown clearly in interference optical microscopy studies of methanol adsorbing into crystals of CrAPO-5, where the hexagonal prismatic crystals are found to possess large sectors that are inaccessible to adsorbate molecules. The existence of barriers to molecular adsorbates at the external surfaces of zeolites has also frequently been reported and can be visualised directly by comparative interference microscopy on as-prepared and also surface-etched ZSM-5 crystals. ... 

See also Diffusion Studied Using NMR Spectroscopy Electromagnetic Radiation Fourier Transformation and Sampling Theory Laser Applications in Electronic Spectroscopy Laser Spectroscopy Theory Light Sources and Optics Neutron Diffraction, Instrumentation Scattering Theory. 

The literature examined here includes three major experimental approaches, namely (i) optical probe diffusion studies, largely made with quasi elastic light scattering spectroscopy (QELSS), to observe diffusion of dilute probe particles, (ii) particle tracking studies in which the detailed motions of individual particles are recorded, and (iii) true microrheology measurements of the driven motion of mesoscopic probes. 

For clarity, the remaining literature in this section is discussed in chronological order. Work by Phillies and Quinlan had been preceded by extensive optical probe diffusion studies of HPC water solutions Brown and Rymden used QELSS to examine 72 nm radius PSL spheres diffusing in solutions of hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), carboxymethylcellulose (CMC), and polyacrylic acid(PAA)(56). The focus was polymer-induced cluster formation, indicated by the substantial decreases in Dp and increases in the second spectral cumulant as seen at very low (0.001 g/g) concentrations of HEC and HPC. These changes were substantially reversed by the addition of 0.15% Triton X-100. The Dp of spheres was reduced by the addition of small amounts of fully-charged pH 9 CMC, but addition of TX-lOO had no effect in CMC solutions. Brown and Rymden also examined sphere diffusion in nondilute polymer solutions. Relatively complex dependences of Dp on concentration were suppressed by the addition of TX-IOO. In the presence of TX-lOO, simple stretched-exponential concentration dependences were observed, but the second spectral cumulant still increased with increasing polymer concentration. 

As mentioned earlier, PAS is a good tool for studying the optical and thermal properties of a sample. Hence, this technique can be applied to fields of dermatological research such as drug detection and drug diffusion studies in skin, and thermal properties and water content of the skin. The optical and thermal properties of the sample are characterized by the optical absorption distance, and the thermal diffusion distance, /Xg. The optical absorption distance is the inverse of the optical absorption coefficient /3. The thermal diffusion length is defined by... 

Zhu X D, Rasing T H and Shen Y R 1988 Surface diffusion of CO on Ni(111) studied by diffraction of optical second-harmonic generation off a monolayer grating Phys. Rev. Lett. 61 2883-5... 

Osborne M A, Balasubramanian S, Furey W S and Klenerman D 1998 Optically biased diffusion of single molecules studied by confocal fluorescence microscopy J. Chem. Phys. B 102 3160-7... 

Many inorganic solids lend themselves to study by PL, to probe their intrinsic properties and to look at impurities and defects. Such materials include alkali-halides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. 

The apparatuses used for the studies of both ammonia synthesis emd hydrodesulfurization were almost identical, consisting of a UHV chamber pumped by both ion and oil diffusion pumps to base pressures of 1 x10 " Torr. Each chamber was equipped with Low Energy Electron Diffraction optics used to determine the orientation of the surfaces and to ascertain that the surfaces were indeed well-ordered. The LEED optics doubled as retarding field analyzers used for Auger Electron Spectroscopy. In addition, each chamber was equipped with a UTI 100C quadrupole mass spectrometer used for analysis of background gases and for Thermal Desorption Spectroscopy studies. 

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