Small glancing angle

Fig. 4.10. Fluorescence signal from small particles or thin films deposited on a silicon substrate used as sample carrier. The intensity was calculated for particles, thin films, or sections ofdiffe-rent thickness but equal mass of analyte, and plotted against the glancing angle f. A Mo-Ka beam was assumed for excitation. Particles or films more than 100 nm thick show double intensity below the critical angle of0.1° [4.21].

When a beam of He+ ions with total energy of 1 -20 keV is incident on metal surfaces at glancing angles (here the angle of the beam with the surface is typically smaller than 10°) the component of the ion velocity perpendicular to the surface is so small that the ions never get closer than —1.5 a.u. from the first atomic layer. For Al, as we showed in Section 2.2, the He-Is level never resonates with the conduction band of the metal, this fact making Auger processes the only ones responsible for neutralization of the incoming He+ ion. 

The basic experimental equipment is the same as in Rutherford backscattering spectrometry (RBS), but it is usual to use the glancing geometry with very small incident angles. 

Measurements on crystalline powders lead to a solution of only relatively small and simple structures Most of the reflections must be resolved, so that intensities can be associated with them unequivocally. Even theoretically, this is often impossible, e.g., when several reflections occur at precisely the same glancing angle. In the cubic crystal system this is so when the sum of the squares of the indices does not permit a single indexing e.g., (221) and (300), where the sum of the squares of the indices is 9 for each case. Measurements on crystalline powders that give unambiguous values for all reflections should be inherently preferable to single-crystal measurements, because many experimental difficulties (e.g.. extinction and absorption) either do not occur at all. or are easier to handle. 

It is very difficult to separate the broadening of a reflection into components due to small crystallite size and to structural defects. Equation (65) implies that PcosO should be constant for all glancing angles, if the broadening results entirely from small crystallite size. Because L is the dimension peipendicular to the diffracting net planes, this statement cannot hold rigorously, except for the diffraction orders of one and the same net plane. If line broadening is the consequence of crystal structure defects alone, the width increases with 0. 

Fig. 39.1 Residual Raman spectra of the H-0 stretching modes of bulk water at room-temperature (solid blue trace), the air-water interface (dashed blue trace), bulk ice at —20 °C (dotted red trace), and the air-ice interface at —15 °C (dashed red trace) detected using glancing angle Raman spectroscopy with insets being the raw data of measurements. The residual Raman spectra were obtained by subtracting the spectrum collected at larger angles (between the surface normal and the reflection beam) from the one collected at small angles (Reprinted with permission from [3]) upon the spectral area being normalized. The side panel shows the skin structure of water...

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