Number-resolved imaging of excited-state atoms using a scanning autoionization microscope

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$^{-2}$.