Karshenboim, Savely G. Laser Spectroscopy, Max Planck Institute of Quantum Optics, Max Planck Society;
Karshenboim, S. G., & Ivanov, V. G. (2005). Improved theoretical prediction for the 2s hyperfine interval in helium ion. Canadian Journal of Physics, 83(10), 1063-1069. doi:10.1139/P05-059.