Measurement of the He ground state lamb shift via the two-photon 11S-21S transition - Bergeson, SD; Balakrishnan, A; Baldwin, KGH; Lucatorto, TB; Marangos, JP; McIlrath, TJ; O'Brian, TR; Rolston, SL; Sansonetti, CJ; Wen, J; Westbrook, N; Cheng, CH; et al. (PHYSICAL REVIEW LETTERS, 1998)