Isotope shifts in the spectrum of helium

Abstract

The iṡotope shift in fourteen singlet lines and four triplet lines of helium was measured interferometrically. Extrapolation of the change of the shift within each series allowed the isotope shift of the terms to be determined. The shifts in the higher 1 D terms were found to show anomalies which are ascribed to perturbations and hyperfine structures. The results of this work in conjunction with measurements by Fred, Tomkins & Brody lead to the conclusion that the values of the isotope shifts given by the theory of Hughes & Eckart are too small throughout by amounts up to 20%.