Incident beam characterization

Photomultipliers are used to measure the intensity of the scattered light. The output is compared to that of a second photocell located in the light trap which measures the intensity of the incident beam. In this way the ratio [J q is measured directly with built-in compensation for any variations in the source. When filters are used for measuring depolarization, their effect on the sensitivity of the photomultiplier and its output must also be considered. Instrument calibration can be accomplished using well-characterized polymer solutions, dispersions of colloidal silica, or opalescent glass as standards. 

The arrangement illustrated in Figure 1 is commonly used for angular characterization of scattered light. The light source is usually a laser. The incident beam may be unpolarized, or it can be linearly polarized with provisions for rotating the plane of polarization. Typically the plane of polarization is perpendicular to the plane of... 

A final practical note involves instrument intensity measurement calibrations. The intensity measurement is self-calibrating relative to the incident beam from the source. However, measurements typically have a dynamic range of 10 -10 , and care must be taken to insure the detection system is linear. A method of calibrating the scatterometer is to characterize a diffuse reflector having a known scattering characteristic. For example, a surface coated with BaS04 makes a nearly Lambertian scatterer, which has a BRDF of 1/Jt at all angles. 

Sample Chamber and Detector. The pressure in the sample chamber is typically 10-6torr, although UHY may be required for some experiments. The samples are usually mounted on a five-axis goniometer, so that a series of samples may be loaded and analysed sequentially. The goniometer can tilt and rotate the samples relative to the direction of the incident beam. Comparing spectra obtained at different incident and exit beam angles provides fuller characterization of the sample composition as a function of depth. The samples can be electrical insulators... 

The light reflected by a powdered solid will consist of a specular reflection component and of a diffuse reflection component. The specular component represents reflection of the incident light by the surfaces of the component particles, and it is characterized by a complete absence of light transmission through the interiors of the particles. By contrast, diffuse reflectance is associated with the radiation that penetrates into the particles to some extent and that then emerges from the bulk solid. This light will exhibit spectral characteristics that are modified from those of the incident beam by the electronic transitions that took place within the solid phase and at the boundaries of the component particles. 

The fact that LEIS provides quantitative information on the outer layer composition of multi-component materials makes this technique an extremely powerful tool for the characterization of catalysts. Figure 4.19 shows the LEIS spectrum of an alumina-supported copper catalyst, taken with an incident beam of 3 keV 4He+ ions. Peaks due to Cu, A1 and O and a fluorine impurity are readily recognized. The high intensity between about 40 and 250 eV is due to secondary (sputtered) ions. The fact that this peak starts at about 40 eV indicates that the sample has charged positively. Of course, the energy scale needs to be corrected for this charge shift before kinematic factors Ef/E-, are determined. 

The advantage of being able to record diffraction intensities over a range of incident beam directions makes CBED readily accessible for comparison with simulations. Thus, CBED is a quantitative diffraction technique. In past 15 years, CBED has evolved from a tool primarily for crystal symmetry determination to the most accurate technique for strain and structure factor measurement [16]. For defects, large angle CBED technique can characterize individual dislocations, stacking faults and interfaces. For applications to defect structures and structure without three-dimensional periodicity, parallel-beam illumination with a very small beam convergence is required. 

Characterization.— The LSFTO powder was calcined at a series of temperatures (1250, 1300, and 1400°C) in air to investigate phase purity and densification behavior. X-ray diffraction (XRD) powder patterns are shown in Fig. 1. The sample is single phase after heating at 1250°C. At the higher sintering temperatures, the lines become sharper and the density increases. The density measured by the Archimedes method was 90.3% relative to theoretical value after annealing at 1400°C for 10 h. The XRD pattern sintered at 1400°C was completely indexed with a cubic unit cell with lattice parameter a = 3.898(8) A and V= 59.2(6) A3. The weak XRD peaks at 31, 43, 55, and 65° 20 are also from the perovskite phase and arise from a small amount of WL radiation in the incident beam. 

In (8) and (9), I is the intensity scattered by one electron, and Sr are the specific surfaces, or surface areas per unit mass of coal, of the macropores and transition pores, respectively the constant C. is proportional to the weak but constant scattering from the micropores b is a constant characterizing the micropore dimensions M and A are respectively the mass of the sample and its cross-section area perpendicular to the incident beam T is the x-ray transmission and a is a constant inversely proportional to the average dimensions of the transition pores. The factor 1/T is included in (9) to take account of the absorption of x-ray in the samples, since (3) was developed under the assumption that the samples were non-absorbing. The transmission T can be expressed—... 

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