  The laser-excitation of Rydberg atoms in ultracold gases is often described
assuming that the atomic motion is frozen during the excitation time. We show
that this frozen gas approximation can break down for atoms that are held in
optical lattices or microtraps. In particular, we show that the excitation
dynamics is in general strongly affected by mechanical forces among the Rydberg
atoms as well as the spread of the atomic wavepacket in the confining
potential. This causes decoherence in the excitation dynamics - resulting in a
dissipative blockade effect - that renders the Rydberg excitation inefficient
even in the anti-blockade regime. For a strongly off-resonant laser excitation
- usually considered in the context of Rydberg dressing - these motional
effects compromise the applicability of the Born-Oppenheimer approximation. In
particular, our results indicate that they can lead to decoherence also in the
dressing regime.
