Precision mapping of electrostatic fields using interference-narrowed Stark resonances

We have exploited the interference narrowing of Stark resonances to map electrostatic fields to a previously unattained level of accuracy and spatial resolution. The technique involves stepwise excitation with tightly focused laser light, an atomic beam, and temporal observation of ionization over a region of interference narrowing. Field inhomogeneities as small as 70 ppm have been detected with spatial resolution of 250m in three dimensions. The method is used here with rubidium atoms at fields of 34 kV/cm, but any alkali-metal atom over a range from a few tens of V/cm to a few tens of kV/cm is appropriate. Ionization rates may be calculated with the WKB quantum-defect method, but the field-mapping results are independent of the theory. Here this mapping technique is used to evaluate two electrode configurations for precision Stark spectroscopy. Field inhomogeneities produced by an electrode with a narrow slit, when properly constructed, are found to be in good agreement with detailed electrostatic calculations. However, discrepancies between field measurements and calculations for a metal mesh over an aperture indicate the presence of unavoidable crinkles of the mesh. We conclude that the slot is the preferred arrangement, and we present calculations that give the field inhomogeneity as a function of the slot size.