Step Scans: The Effect of Step Size
This 3D graph below shows the "five fingers" region of quartz between 67° 2θ and 69° 2θ using a 1° divergence aperture. The scans were taken with a 1 second dwell time in step scan mode. In step scan mode the goniometer moves by discrete angular steps, Δ2θ, but then stops and acquires data for the length of the dwell time. Then the goniometer steps another Δ2θ, acquires for the dwell time, and so on. The five finger region includes the quartz (212), (203) and (301) peaks and is in a range of 2θ that the Kα1-Kα2 doublet is resolvable. It is called the "five finger" region because the Kα2 (203) peak overlaps with the Kα1 (301) peak producing five instead of six peaks from three doublets. The effect of step size, Δ2θ, was studied. The step sizes in the graph above are: 0.200° (front), 0.150°, 0.100°, 0.050°, 0.020°, 0.004° and 0.002° (back). In the case of continuous scans, the diffracted data is integrated over an angular range of 2θ. In the case ofcontinuous scans, this integrated angular range is a function of step size, Δ2θ as the goniometer is continuously slewing at a constant angular velocity, and thus the intensity of the peaks is step-size dependent. In the case of step scans, the intensities are not step size dependent and are entirely determined by the dwell time. As such, the step size determines only the granularity of the data. While the diffraction patterns in the 3D graph above have different number of points-- all those points have the same intensity.
In the case of continuous scans, the step size, Δ2θ, has no effect on the scan time as it is determined by the total angular range and the slew rate in °/min. In the case of step scans, each step takes requires some "travel time" to accelerate and decelerate to the acquisition position as well as the dwell time, and thus scan time in step mode is step size dependent. As in the study of continuous scans, the larger step sizes prevent even the identification of all of the peak positions, while below a step size of 0.100° there is only a minor effect on the resolution of peak positions and FWHM's. However, since extra steps cost extra time in step scan mode, a step size, Δ2θ, that is appropriate for the diffracted peak widths desirable as it saves time. For example, in the case of the 0.002° Δ2θ scan above, the position of the quartz (212) peak was found to be 2θ = 67.702 ± 0.001°, while the 0.020° Δ2θ scan yielded 67.699 ± 0.003°. That is not much of a change in measured certainty for a 8X increase in acquisition time.
copy from : http://xrd-cmmp.blogspot.my/
Posted by Dr. Eric Lochner at 12:17 PM
