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We investigate collisional shifts of spectral lines involving excitedhydrogenic states, where van der Waals coefficients have recently been shown tohave large numerical values when expressed in atomic units. Particular emphasisis laid on the recent hydrogen 2S-4P experiment (and an ongoing 2S-6Pexperiment) in Garching, but numerical input data are provided for othertransitions (e.g., involving S states), as well. We show that the frequencyshifts can be described, to sufficient accuracy, in the impact approximation.The pressure related effects were separated into two parts, (i) related tocollisions of atoms inside of the beam, and (ii) related to collisions of theatoms in the atomic beam with the residual background gas. The latter containsboth atomic as well as molecular hydrogen. The dominant effect of intra-beamcollisions is evaluated by a Monte-Carlo simulation, taking the geometry of theexperimental apparatus into account. While, in the Garching experiment, thecollisional shift is on the order of 10 Hz, and thus negligible, it candecisively depend on the experimental conditions. We present input data whichcan be used in order to describe the effect for other transitions of currentand planned experimental interest.

Pressure Shifts in High-Precision Hydrogen Spectroscopy: II. Impact Approximation and Monte-Carlo Simulations