An atom interacting with a quantized electromagnetic field in a cavity with
time-dependent parameters is considered, Variation of the cavity parameter
s results in nonstationary dynamics of the field which leads, in turn. to e
xcitation of the atom, even if photons were initially absent in the cavity.
We distinguish three mechanisms of such excitation: excitation due to abso
rption of real photons created by the dynamical Casimir effect, excitation
due to absorption of virtual photons during the transient process, and exci
tation due to nonadiabatic parametric modulation of the atomic Lamb shift.
The last mechanism has no relation to the dynamical Casimir effect and thus
should be considered as a new vacuum QED effect. Normally all these three
mechanisms give a contribution to the amplitude of the atom excitation and
are accompanied by the creation of photons. Therefore the presence of an at
om in the cavity alters the average number of created photons in comparison
with the case of an empty non-stationary cavity. Our consideration is base
d mainly on a simple model of a two-level atom interacting with a single mo
de of quantized electromagnetic field. However, our results are qualitative
ly valid for more realistic models.