  We study the behavior of a quantum particle, trapped in localized potential,
when the trapping potential starts suddenly to move with constant velocity. In
one dimension we have reproduced the results obtained by Granot and Marchewka,
Ref. \cite{Granot09}, for an attractive delta function, using an approach based
on a spectral decomposition, rather than on the propagator. We have also
considered the cases of P\"oschl-Teller and simple harmonic oscillator
potentials (in one dimension) and to the hydrogen atom (in three dimensions).
In this last case we have calculated explicitly the leading contribution to the
ionization probability for the hydrogen atom due to a sudden movement.
