EPR spectrum

You place your one-crystal in the cavity (one-crystal because for powder you don’t need to rotate as all orientations are present at once) and apply an external field. Simultaneously microwaves with a constant frequency are reflecting on the cavity. You scan over the external magnetic field to find the peaks. After this scan is finished you slightly rotate the sample (using a goniometer) in the cavity so that the orientation is different and you scan again to find the peaks. And repeat this to satisfaction of angular resolution. Each position of the peak corresponds to a g-factor which can then be plotted in a figure like here.