Step scan data collection

The majority of experimental powder diffraction patterns in the following five examples were collected using a step scan data collection method on a Scintag XDS2000 system equipped with a Ge(Li) solid-state detector cooled with liquid nitrogen. 

Birknes, B., and Hansen, L. K. A method for collection of step-scan data on a CAD 4 diffractometer homogeneity of the monochromated X-ray beam. J. Appl. Cryst. 16, 11-13 (1983). 

You are using Fe Ka radiation to collect powder diffraction data employing powder diffractometer C (see problem 4). After several quick scans, you established that the receiving slit with the aperture of 0.03° results in both acceptable resolution and intensity. Bragg peaks appear to have a full width at half maximum between 0.4 and 0.5° 20. What is the largest allowable step during data collection and why ... 

Data collected during these process variable scans indicate that more C02 than expected was being formed (Figure 4). Selectivity to C02 reached a maximum of 5-10% at about 90-95% conversion. At higher conversions, the C02 level is reduced by reverse shift and subsequent methanation of CO or by direct methanation of C02. This selectivity to C02 can be eliminated by cofeeding small amounts of C02 (3-5%). Since multiple C02 absorbers are required in the commercial SNG plant, one or more could be relocated downstream of the methanation step. This could offer some economic advantages since C02 absorption would now occur at higher concentration and pressure and at lower total gas flow. 

In conventional nanosecond pump-probe dispersive TRIR experiments, also described previously, kinetic data are collected at one frequency at a time. These data can then be used to construct a series of time-resolved IR spectra. Thus, in the dispersive experiment kinetic data are used to construct spectra, and in the step-scan experiment spectral data are used to derive kinetics. 

X-ray diffraction was done using a Siemens D-500 diffractometer utilizing CuKa radiation (1.406 A). The data were collected as step scans, with a step size of 0.05° 29 and a count time of 2 s/step between 10° and 80° 29. 

In practice, it often happens that the available range of scan rates restricts data collection within only one of the two limiting regions, corresponding to electron transfer or follow-up reaction control, and in the intermediate region. The use of transition curves such as those in Figure 2.5 nevertheless allows characterization of the two steps. 

They have a good signal-to-noise ratio, due to rapid data collection. This is because all of the data S(v) are practically measured at the same time by the detector (the x scan is very fast), at variance with dispersive systems (such as the spectrophotometer discussed in Section 1.3), for which the frequency must be changed step by step... 

Fig. 1.2. Line-scan image and gray-scale image. The two images represent the same set of data collected by STM on a Si(lll)-7X7 surface with steps. Dimensions 100 X 125 A . (Reproduced from Becker et al.. 1985a, with permission.)...

Several improvements to the basic data collection algorithm have been made. Perhaps the most significant is the use of the step-scan technique, versions of which were developed in 1969 for our computerized diffractometer, and simultaneously elsewhere. 

A generic algorithm of data collection in the step scanning mode is shown in the form of a flowchart in Figure 3.42. It includes the following sequence of events ... 

Figure 5.5. A fragment of the diffraction pattern collected from a LaNi4 ssSno.is powder on a Rigaku TTRAX rotating anode powder diffractometer using Cu Ka radiation. The data were collected in a step scan mode with a step 0.02° of 20 and counting time 4 sec. As explained below (see Table 5.2 and Table 5.4, respectively), the two sets of vertical bars indicate locations of Bragg peaks calculated using the first (the upper set of bars) and the second (the lower set of bars) approximations of the unit cell dimensions.

Figure 5.10. The x-ray powder diffraction pattern of U3Ni6Si2 collected on an HZG-4a powder diffractometer using filtered Cu Ka radiation. The data were collected in a step scan mode with a step 0.02 of 20 and counting time 25 sec. The ASCII data file with the diffraction data is available on the CD, file name Ch5Ex03 CuKa.xy. Data courtesy of Dr. L.G. Akselrud. When compared, for example, with Figure 5.5 and Figure 5.9, the increased background is noteworthy, which occurs as a result of its incomplete elimination when using a P-filter.

Figure 6.15. The observed and calculated powder diffraction patterns of CeRhGea after all parameters were refined in the same approximation as in the previous example. The powder diffraction data were collected from a ground sample of CeRhGes using Mo Ka radiation on a Rigaku TTRAX rotating anode diffractometer. The divergence slit was 0.38° the receiving slit was 0.03°. The experiment was carried out in a step scan mode with a step 0.01° and counting time 4 sec per step. The inset illustrates an inadequate asymmetry approximation.

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