  We consider a system formed by an array of Bose-Einstein condensates trapped
in a harmonic potential with a superimposed periodic optical potential.
Starting from the boson field Hamiltonian, appropriate to describe dilute gas
of bosonic atoms, we reformulate the system dynamics within the Bose-Hubbard
model picture. Then we analyse the effective dynamics of the system when the
optical potential depth is suddenly varied according to a procedure applied in
many of the recent experiments on superfluid-Mott transition in Bose-Einstein
condensates.
  Initially the condensates' array generated in a weak optical potential is
assumed to be in the superfluid ground-state which is well described in terms
of coherent states. At a given time, the optical potential depth is suddenly
increased and, after a waiting time, it is quickly decreased so that the
initial depth is restored. We compute the system-state evolution and show that
the potential jump brings on an excitation of the system, incorporated in the
final condensate wave functions, whose effects are analysed in terms of
two-site correlation functions and of on-site population oscillations. Also we
show how a too long waiting time can destroy completely the coherence of the
final state making it unobservable.
