  We show that a Bose-Einstein condensate emits atoms, if either the condensate
wave function, or the scattering length of the atoms depends strongly on time.
Moreover, the emission process is coherent and atoms can oscillate back and
forth between the condensate and the excited states. Inspired by recent
experimental results, we present results of simulations of the response of a
Bose-Einstein condensate to a very rapid change in the scattering length. The
possibility of molecule formation is also discussed.
