Background Determination and Subtraction

Depending on the measurement arrangement, every powder diffraction pattern generally also contains more or less contributions from statistical noise which causes 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! uses the method of E.J. Sonneveld and J.W. Visser (described in J. Appl. Cryst. 8, 1 (1975)) to perform this task.

The contribution of the background radiation (simply called “the background”) is first determined automatically during the import of the raw (profile) diffraction data, using the method just mentioned. It is displayed as an (by default) orange colored curve with small squares (control points) at the bottom of the raw (profile) diffraction data right after the import.

There may be cases though in which the automatic determination does not give a satisfactory result, so that you would like to correct it. This should be done before running the peak searching command! Maybe the automatically determined background does not completely detect all features, either because the background is too “rigid” (in which case amorphous “humps” are not fully covered, leading to surplus peaks when using the automatic peak searching), or too flexible (in which case profile areas that belong to the peaks may be covered (i.e. hidden) by the background).

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 in your profile data using the corresponding command 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 the alpha2 stripping 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.