Karshenboim, Savely G. Laser Spectroscopy, Max Planck Institute of Quantum Optics, Max Planck Society;
Lee, R. N., Milstein, A. I., & Karshenboim, S. G. (2006). Corrections to the energy levels of a spin-zero particle bound in a strong field. Physical Review A, 73(1): 012505. doi:10.1103/PhysRevA.73.012505.