Background Determination and Subtraction

Depending on the measurement arrangement, every powder diffraction pattern generally also contains more or less contributions which cause the base-line of the diffraction pattern to deviate from the "ideal" flat line at zero intensity. This "background" has to be determined as accurately as possible, in order to be able to determine the intensities of the peaks during the peak searching.

Match! determines the contribution of the background radiation (simply called "the background") automatically during the import of the raw (profile) diffraction data, using one of the methods described below. The background is by default displayed as an orange-colored curve with small squares (control points) at the bottom of the raw (profile) diffraction data right after the import.

Match! offers three different methods for background determination:

You can select the default method for background determination on the "Raw data" tab of the "Options" window.

Once the background has been determined using one or the other method just mentioned, you can also modify the background curve manually:

The easiest way to do so is to use one of the commands "Set background to zero", "Two points" or "Recalculate background" from the "Background" sub-menu of the "Pattern" menu. This will simply apply the corresponding "built-in" method for background determination mentioned above. ("Recalculate background" corresponds to the automatic determination using the Sonnefeld-Visser method).

If the automatically determined background curve is too high or too low, you can either press <Ctrl+Shift+F2> (which will raise the relative background intensity by 0.5) or <Ctrl+Shift+F3> in order to lower the background by the same amount. An automatic correction is also available which will be performed at pattern import.

If the background curve does not follow the raw data close enough (or too close), you can correct these issues by pressing <Ctrl+F2> (if you would like to increase the background flexibility) or <Ctrl+F3> (if the background shall be more rigid (cover less of the peak sockets), i.e. in order to decrease the flexibility) until the visual agreement between the orange curve and the estimated background is acceptable.

It is also possible to modify the shape of the background curve manually, simply by dragging around control points of the background curve using the mouse. It is also easily possible to add more control points or delete existing ones just by clicking the right or left mouse button if the mouse cursor is at the corresponding position.

In order to do so, please make sure that the background curve is currently displayed. You can toggle the displaying of the background curve e.g. by right-clicking in the diffraction pattern and selecting "Background" from the context menu that opens.

When the background is displayed, please note the small squares on the curve. These are the so-called "background control points". You can shift these points by first moving the mouse on top of them, pressing the left mouse button, and moving the mouse around.

If you would like to add more control points (e.g. to be able to model a certain range in the background curve more accurately), you can do so by first moving the mouse to the desired position on the background curve. The mouse cursor will change (a "+" sign will appear on the top left of the cursor); you can now press the left mouse button in order to add a control point at the corresponding position of the background curve.

In order to delete a control point, simply move the mouse cursor on top of it, then press the right mouse button.

Please note: If you intend to strip the alpha2 contribution or increase the resolution of your profile data using the corresponding commands from the "Pattern"-menu, please make sure to do so first before you modify the background manually, because your manually defined background will be overwritten by these functions function.

In order to subtract the background, either select the "Subtract background" command from the "Pattern" menu or press the corresponding button in the main toolbar at the top.