Spectroscopy, angle-dependent

For IR spectroscopy, the process of interest is absorption. Polarization and angle-dependent measurements are useful when using the transmission geometry. 

They studied the chemistry of oxidized Si(lll) surfaces treated at two concentrations of the silane in trichloroethylene solution using angle-dependent X-ray photoelectron spectroscopy (XPS or ESCA). Although these are non-aqueous adsorption studies, sufficient surface silanol or adsorbed water is present for complete hydrolysis to occur because no trace of chlorine is seen in the XPS spectra. The two concentrations studied were 1% v/v, termed saturated, and <1/400% v/v, termed dilute. They lead to two distinct types of molecular bonding to the surface. C( 1 s) XPS spectra of these two situations are shown in Fig. 3. 

Here we discuss the application of low energy photoelectron transmission spectroscopy (LEPS) as a means for investigating the electronic properties of OOTF. In a typical experiment photoelectrons are ejected from the conductive substrate and after being transmitted through the adsorbed layer, the energy (and or angle) dependent electrons flux is measured as a function of incident photon energy, molecular film thickness, adsorbate and substrate types and temperature. 

XPS spectra were recorded using unmonochromatized Mg K radiation (1253.6 eV), and an unmonochromatized He-resonance lamp was used for ultraviolet photoelectron spectroscopy (UPS). XPS spectra were taken with an analyzer resolution of 0.2 eV, and the net resolution measured as the full width at half-maximum (FWHM) of Au 4f(7/2) was 0.9 eV. The spectrometer is of our own construction and is, e.g., designed to provide optimum angle-dependent XPS or XPS(0) (12,l4). For high 5-values, the photoelectrons leave the sample surface near the grazing angel, and due to the limited escape depth of the electrons, this is a "surface sensitive" mode. In the "bulk sensitive" mode, for low 0-values, the photoelectrons exit near the surface normal, and hence more information from the "bulk" of the sample is obtained (15). 

The wavelength and angle dependences of the depth of penetration, d, were given earlier (Equation (3.22)), and this principle is used in the MIR for the sampling technique of ATR spectroscopy. Figure 3.13(c) illustrated this, and, for typical refractive indices of fibre core and sample of 2.5 and 1.5, respectively, and an angle of incidence of 45°, the depth of penetration of the evanescent wave is about 0.152. The effect of the evanescent wave on fibre-optic spectroscopy may be illustrated by the example of plastic-clad silica (PCS) fibre optics for remote spectroscopy in the NIR spectral region, as discussed below. 

A type of angle-dependent x-ray photoemission spectroscopy was used to investigate the molecular orientation at the surface of sulfonated polystyrene as a function of reaction depth. A model based on these measurements indicates that at a critical sulfonation depth the aliphatic hydrocarbon backbone becomes exposed preferentially at the surface. These results are consistent with surface energy and tribo-electric charging measurements, which also reveal the effects of associative interactions in the form of conversion dependencies. 

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