Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic   Hydrogen
  We consider the excitation dynamics of the two-photon \sts transition in a beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the impact of ionization damping on the transition line shape, caused by the possibility of ionization of the 2S level by the same laser field. Using a Monte-Carlo simulation, we calculate the line shape of the \sts transition for the experimental geometry used in the two latest absolute frequency measurements (M. Niering {\it et al.}, PRL 84, 5496 (2000) and M. Fischer {\it et al.}, PRL 92, 230802 (2004)). The calculated line shift and line width are in excellent agreement with the experimentally observed values. From this comparison we can verify the values of the dynamic Stark shift coefficient for the \sts transition for the first time on a level of 15%. We show that the ionization modifies the velocity distribution of the metastable atoms, the line shape of the \sts transition, and has an influence on the derivation of its absolute frequency. 
N. Kolachevsky, M. Haas, U.D. Jentschura, M. Herrmann, P. Fendel, M.   Fischer, R. Holzwarth, Th. Udem, C.H. Keitel, T.W. Ha...