Peak Searching

The most important part of the raw data processing is the searching for diffraction peaks, since the peak positions and intensities are the actual input to the normal (not profile fitting) search-match calculation (i.e. the qualitative phase analysis). Match! uses the well-known first- and optionally the second-derivative method to perform this task.

The peak searching can be carried out manually by selecting the "Peak search" command from the "Pattern" menu (or using the corresponding button ), or implicitly during the automatic raw data processing. Optionally, profile fitting can be run immediately after peak searching to get even more accurate peak data.

If a mixed radiation (containing alpha1 and alpha2 contribution) has been used in the diffraction experiment, you can activate the option "Operate alpha2 peaks automatically" on the "Raw data" page of the "Options" window (option is active by default). If you activate it, Match! will explicitly add the alpha2-peaks during peak searching if a mixed (alpha1+2) wavelength is used, even if only the corresponding alpha1-peak is detected.

Before running the peak searching we strongly recommend to check and maybe correct the background. You should always keep in mind that if the background is too high in certain parts of the diffraction pattern, this may prohibit the detection of peaks in this area! You could e.g. try to reduce the background flexibility by pressing <Ctrl+F3> one or two times (<Cmd+F3> on the Mac).

If the raw diffraction data are very rugged, you could try to smooth your raw data before running the peak searching.

FWHM

If you use the normal peak searching functionality, Match! will try to determine the FWHM (Full Width at Half Maximum) separately for each peak automatically. If this is not possible, it will use the corresponding default value (which can be adjusted in the "Automatic Raw Data Processing Options" dialog). If you would like to obtain more accurate FWHM values, you should run profile fitting after the peak searching (or activate the corresponding option on the "Raw data" page of the "Options" window).

Integrated Intensities

Normally, the intensity values that are calculated during the peak searching procedure are integrated intensities, taking into account both the height as well as the FWHM of each peak. You can view the integrated intensity values in the peak list on the upper right-hand side. An exception to this is the relative intensity scaling of the vertical axis, in which case the integrated intensity values are scaled to a maximum value of 1000.0.

Peak Searching Sensitivity

One task associated with peak searching is to decide which "hump" in the raw data corresponds to a Bragg peak and which is an artifact or noise. Obviously, there has to be a certain parameter "sensitivity" using which the software can make this decision.

When peak searching (in any variant just described) is carried out for the first time, a default sensitivity is used which takes the "noise" found in the raw diffraction data into account. This default sensitivity has been chosen to obtain good results with a large variety of diffraction patterns. However, it may nevertheless be necessary to adjust the sensitivity for your special data. Note that you can also modify the default peak search sensitivity value in the "Automatic Raw Data Processing Options" window.

Note:

• If one or more phases seem to be missing (typically minor phases), you should try to increase the peak searching sensitivity as described above. Besides this, you should also deactivate the '3 strongest peaks' condition for the search-match calculation, because a peak can easily be missing for minor phases which might prevent the phase from being listed in the candidate list if this option is active.
• If some peaks are not detected, this may be caused by the fact that when detecting a new peak (based on a zero first derivative value), Match! searches for a maximum in the raw data within two times the FWHM. This can cause shoulder peaks to be neglected if the default FWHM value is relatively large. You should adjust the default FWHM value on the "Raw data" tab of the "Options" dialog (maybe you have to deactivate the corresponding "Automatic" option in order to be able to modify the default FWHM value).
• If peak positions are not determined exactly, this may be caused by a relatively large step width in your raw/profile data. In order to fix this problem, you could try to increase the resolution (i.e. decrease the step width) by interpolation, using the menu command "Pattern/Increase resolution...". You should start by trying a small factor (e.g. 2 or 3) first, because otherwise the noise may be "fixed" (i.e. fluctuations are described by too many data points, so they are no longer detected as "random noise" but as a "signal"). Repeat the peak detection after increasing the resolution.
• The best way to cope with missing or surplus peaks is still to visually inspect the diffraction pattern on your own and to decide about the presence or absence of a peak guided by both scientific experience and intuition. This can be done in a very comfortable way using Match!'s zoom or zoom/track and peak editing funcionalities. A careful check and probably correction of the peaks will safe a lot of time that can be wasted in performing a search-match on bad peak data!