Measurement of High-Angular-Momentum Fine Structure in Helium: An Experimental Test of Long-Range Electromagnetic Forces

Abstract

Fine-structure intervals separating the $n=10\mathrm{}$, $L=4, 5, 6, \mathrm{and} 7$ 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.