  We report on a scanning microscopy technique for atom-number-resolved imaging
of excited-state atoms. A tightly focused laser beam leads to local
autoionization, and the resulting ions are counted electronically. Scanning the
beam across the cloud builds up an image of the density distribution of excited
atoms, with access to the full counting statistics at each spatial sampling
point and an overall detection efficiency of 21 %. We apply this technique to
the measurement of a spatially inhomogeneous electric field with a spatial
resolution of 50 ?m and a sensitivity to electric field gradients of 0.04 V
cm$^{-2}$.
