  We consider an atom interacting with a quantized electromagnetic field inside
a cavity with variable parameters. The atom in the ground state located in the
initially empty cavity can be excited by variation of cavity parameters. We
have discovered two mechanisms of atomic excitation. The first arises due to
the interaction of the atom with the non-stationary electromagnetic field
created by modulation of cavity parameters. If the characteristic time of
variation of cavity parameters is of the order of the atomic transition time,
the processes of photon creation and atomic excitation are going on
simultaneously and hence excitation of the atom cannot be reduced to trivial
absorption of the photons produced by the dynamical Casimir effect. The second
mechanism is "shaking" of the atom due to fast modulation of its ground state
Lamb shift which takes place as a result of fast variation of cavity arameters.
The last mechanism has no connection with the vacuum dynamical Casimir effect.
Moreover, it opens a new channel of photon creation in the non-stationary
cavity. Nevertheless, the process of photon creation is altered by the presence
of the atom in the cavity, even if one disregards the existence of the new
channel. In particular, it removes the restriction for creation of only even
number of photons and also changes the expectation value for the number of
created photons. Our consideration is based on a simple model of a two-level
atom interacting with a single mode of the cavity field. Qualitatively our
results are valid for a real atom in a physical cavity.
