1S–3S cw spectroscopy of hydrogen/deuterium atom Pauline Yzombard, Simon Thomas, Lucile Julien, Francois Biraben and Francois Nez Eur. Phys. J. D, 77 2 (2023) 23 Published online: 08 February 2023 DOI: 10.1140/epjd/s10053-023-00605-9