Fast Beam Measurements of the 10D-10F Fine Structure Intervals in Helium.

Four fine structure intervals separating the n = 10, L = 2 and 3 Rydberg states of helium have been measured using a fast beam microwave-optical resonance technique. The relative positions of the states are determined to a precision of about 16 kHz. Previous measurements of higher-L intervals in n = 10 helium have disagreed with high-precision calculations in a very systematic way, while the previous measurements of the L = 2 to L = 3 intervals agreed precisely with those calculations. These measurements were undertaken in an attempt to resolve this puzzle. In the course of these experiments, two powerful direct methods of determining the size and spatial extent of stray DC electric fields inside the interaction region were developed. The lack of knowledge about the stray field environment in the interaction region was a strong constraint on the precision of previous measurements. We find, however, agreement with the previous measurements of these intervals. The experimental result for the mean 10D-10F interval in helium is 14560.655(11) MHz, in good agreement with both the previous experimental result of 14560.651(34) MHz and the theoretical value of 14560.655(1) MHz.