Measurement of high-angular-momentum fine structure in helium: An experimental test of long-range electromagnetic forces

Fine-structure intervals separating the n=10, L=4, 5, 6, and 7 manifolds of helium have been measured in a fast atomic beam with precision sufficient to resolve predicted vacuum-fluctuation corrections to the structure. We find the intervals (in the absence of spin) to be as follows: 10G-H, 490.990(10) MHz; 10H-I, 157.068(13) MHz; 10I-K, 60.818(10) MHz. The results set an experimental limit on the contributions of retarded long-range interactions that is comparable to their expected size. © 1984 The American Physical Society.