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