XPEEM images of a SAW at bias voltage, VB, of (a) −3 V and (b) +0.5 V. The photon energy (hν = 848 eV), the total exposure time (10 s) and the size (∼30 µm × 23 µm) are the same for both. In the lower left and upper right corners of the images the same alignment markers are visible as dark squares. (c) Bias voltage, VB, scans for the centres of the bright and dark stripes in the image of part (a). The solid and dashed vertical lines represent the conditions at which the images in (a) and (b) were recorded. (d) Splitting between the local spectra as a function of the applied SAW excitation amplitude; the blue line is a guide to the eye.