Step-scan mode

X-ray absorption spectroscopic measurements were carried out at the storage ring DORIS III (HASYLAB DESY, Hamburg, Germany) at the EXAFS II beam line, which was equipped with a Si (111) double-crystal monochromator. All spectra were recorded at room temperature in a step-scanning mode. For data analyses the program WinXAS [17] was used. 

Figure 1.3 Modes of the mirror scanning in interferometry (a) continuous-scan mode and (b) step-scan mode.

While an intensity profile at the detector as a function of retardation may be acquired in a step-scan mode, two major drawbacks affect this method of interferogram acquisition. First, the mirror(s) requires stabilization times with mirror inertia and time constants of the control loop determining this parameter in achieving a given optical retardation. Second, additional hardware and control mechanisms need to be incorporated into the spectrometer, thus increasing instrument cost and complexity. In certain cases, however, the utility of a step-scan instrument justifies this additional expense. Historically, the step-scan approach was favored with slow detectors. With the advent of fast detectors and electronics, step-scan interferometry became... 

The sensitivity and detection limits of an analytical technique are determined by the SNR of the measurement, an important metric for assessing both the instrumental performance and analytic limits of the spectral measurement. Following typical analytical practices, 3 and 10 times the noise have been suggested as limits of detection and of quantification for IR spectroscopy, respectively. The performance of interferometers in the continuous-scan mode, which is simpler compared with that of the step-scan mode, has been analyzed well. The SNR of a spectrum measured using a Michelson interferometer is given by12... 

A suitable description of a peak for advanced Scherrer analysis uses the expression in terms of the centroid and the variance of a peak. For the possible computation of these values, the explicit definitions are given in Equation (9) for the centroid and in Equation (10) for the variance expressed in units of the 20 scale. Conventions for a peak profile measured in a step-scan mode are as follows 20 = position of a peak increment, 1(20) = the background corrected intensity in cps for the increment, A20 = width of the increment ... 

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 3.44. A set of x-ray powder diffraction patterns collected from the LaNi4,g5Sno 15 powder (see the inset in Figure 3.32) on a Rigaku TTRAX rotating anode powder using Mo Ka radiation. Goniometer radius R = 285 mm Divergence slit DS = 0.5° Receiving slit RS = 0.03° flat specimen diameter d = 20 mm. Step scan mode with steps 0.005, 0.01,0.02, 0.03, 0.04 and 0.05°. An automatic variable scatter slit was used to reduce the background.

The two most important parameters in a continuous scan, which are defined by the user, are the sampling interval (step), s, and the angular velocity (scan rate), r. The sampling step is equivalent to the step size in the step scan mode. Everything said about the size of the step in the previous section, therefore, applies to the sampling step during the continuous scan. The two parameters, i.e. counting time, t, in the step scan and the scan rate, r, in the continuous scan are related to one another as follows... 

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 5.17. Powder diffraction pattern of Fe7(P04)s collected on a Scintag XDS2000 diffractometer using Cu Ka radiation in a step scan mode with A20 = 0.02° and counting time 30 sec. The three sets of vertical bars illustrate the following top - positions of the observed Bragg peaks, middle - positions of Bragg peaks calculated using ITO solution No. 1 (correct), and bottom - the same calculated using ITO solution No. 2 (incorrect) both solutions are listed in Table 5.24. Filled circles indicate unobserved reflections and filled triangle indicates the only observed reflection below 20 = 20°, which was left unindexed in the solution No. 2.

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.

Figure 6.23. Powder diffraction pattern collected from the NiMn02(OH) powder using Cu Ka radiation on a Scintag XDS2000 diffractometer. The experiment was carried out in a step scan mode with a step 0.02° and counting time 30 sec per step. The vertical bars indicate calculated positions of the Kai components of all possible Bragg reflections. The inset shows the scanning electron microscopy image of peuticle morphology in the as-received state.

The experimental powder diffraction pattern was collected on a rotating anode Rigaku TTRAX powder diffractometer using monochromatized Mo Ka radiation from 5 to 50° 20 in a step scan mode with a 0.01° step and counting time of 10 sec/step. The following parameters were employed at the beginning of this refinement ... 

A Philips X-ray diffractometer using the step-scan mode and Cu Ka radiation was used to obtain the XRD patterns in the range of 36 to 44° of 29. The step size used was 0.02° of 29 with a counting time of 100 s per step. Comparison of XRD results with transmission electron microscopy results showed that at the above scan rates the Pt 111 line at 39.8° was readily detected for samples containing Pt crystallites >2.0 nm even for Pt contents of 0.5 wt%. 

Such FPA detector setups were first used by the group of Lauterbach for the parallel characterization of solid samples and the product gas stream from catalytic reactors [18,19]. These authors also changed the mode of operation from the previously used step-scan mode to the rapid scan mode which made it possible to even record transient processes [20,21]. The group of Lauterbach was also the first to apply FPA IR spectroscopy to a problem from zeolite science, even if it was only in form of a feasibility study. They investigated the adsorption of CO on Cu-ZSM-5 and on Pt/Si02 in order to prove that it would be possible to detect the absorption bands of adsorbed species [19J. Since experiments were carried out at room temperature, bands for CO on the Cu-ZSM-5 would be expected to have very low intensity, and indeed, no spectra for CO on this solid were shown. The band of CO on the noble metal, on the other hand, could clearly be detected without problems, and a signal-to-noise ratio not much different from that obtained for a conventional experiment. 

Phase compositions were measured by X-ray diffraction using Japanese D/MAX 2500VB instrument in a step-scanning mode with Ni-filtered Cu-Ka as the radiation source and the radiation is over a range of 10 80°. The volume fractions of tetragonal zirconia (K,) are calculated by the following equations [18] ... 

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