Calculation of the 1s–2s two-photon excitation cross-section in atomic hydrogen G. Çelik, E. Çelik and H. Ş. Kiliç Eur. Phys. J. D, 50 3 (2008) 237-240 Published online: 05 December 2008 DOI: 10.1140/epjd/e2008-00233-9