Beams effects

Use of the precession technique should promote a decrease of multi-beam effects and an improvement of the situation with the inaccuracy of orientation. In this case we will have full set of orientations along the surface of cone and the diffraction pattern will be the sum of all patterns for every direction of the beam. 

In addition to the positional and thermal parameters of the atoms, least-squares procedures are used to determine the scale of the data, and parameters such as mosaic spread or particle size, which influence the intensities through multiple-beam effects (Becker and Coppens 1974a, b, 1975). It is not an exaggeration to say that modern crystallography is, to a large extent, made possible by the use of least-squares methods. Similarly, least-squares techniques play a central role in the charge density analysis with the scattering formalisms described in the previous chapter. 

Surface Tool Concerns. Because surface chemistry was of prime interest in the McIntyre study, XPS was chosen as the technique to use. Often AES can provide chemical information, but previous XPS work had shown easily identifiable oxidation states of U2O5 and l Og suggesting that XPS was a good choice and avoiding possible electron beam effects that AES might induce. 

In this study, plasma fluorinated polyimide films of various thicknesses were exposed to a 2 MeV He2+ ion beam at different doses to determine beam effects. A thick PTFE film, irradiated under similar conditions, was also examined surface changes and fluorine loss during RBS analysis were compared. 

Beam Effects, Surface Topography, and Depth Profiling in Surface Analysis (Methods of Surface Characterization), Czanderna, A. W Madey, T. E. Powell, C. J. eds.. Plenum Press New York,... 

Bermudez VM. (1999) Low-energy electron beam effects on poly(methyl methacrylate) resist films. / Vac Sci Technol S 17 2512-2518. 

U. Diebold, in Specimen Handling, Treatments, Beam Effects and Depth Profiling, edited... 

Formation gives clusters of such a size as to be detected by standard techniques for colloidal solutions such as Tyndall s beam effect where scattering of light by the solution occurs. 

Figure 7-5 Illustration of the Gaussian beam effect on intensity measurements by LDA and its effect on the effective cross-section of the measurement volume.

For a simultaneous determination of fluid and particle velocity by LDA the fluid flow has to be additionally seeded by small tracer particles which are able to follow the turbulent fluctuations. The remaining task is the separation of the Doppler signals resulting from tracer particles and the dispersed phase particles. In most cases this discrimination is based on the scattering intensity combined with some other method in order to reduce the error due to the Gaussian beam effect The discrimination procedure introduced by Durst (1982) for example, was based on the use of two receiving optical systems and two photodiodes... 

Figure 7-27 Illustration of Gaussian beam effect (a) and slit effect (b)...

Correct particle size measurements, especially for large particles which have the highest contribution to the local mass concentration. Therefore, sizing errors due to the Gaussian beam effect are a potential source of erroneous concentration measurements. 

Just to complete the discussion on measurement volume size, smaller volumes will cause systematic errors in particle sizing (Gaussian beam effect), as discussed in the following section. Also in multi-component LDA/PDA systems, very small measurement volumes, say below 50pm, become difficult to align on top of one another. Therefore the conclusion is that some optimization is necessary in choosing the measurement volume size. 

For chemical effects in the surface water, the only realistic possibility is to renew the water from time to time. In the case of a cell with a membrane film cover, this requires movement of the membrane and flushing, which probably will require a retuning of the sample position. For the closed chamber vapor-pressure control approach, the constant re-equilibrium of the thin layer of water on the surface should be sufficient if the vapor volume is satisfactorily large compared to the surface water volume. Beam effects on surface species themselves have also been observed, with reduction of both metals and anion complexes. If samples are studied ex situ in air, ozone production can cause oxidation. These effects will be most severe with third generation sources, and need to be taken quite seriously in the design of experiments. 

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