  We investigate (non-relativistic) atomic systems interacting with quantum
electromagnetic field (QEF). The resulting model describes spontaneous emission
of light from a two-level atom surrounded by various initial states of the QEF.
We assume, that the quantum field interacts with the atom via the standard,
minimal-coupling Hamiltonian, with the $A^2$ term neglected. We also assume,
that there will appear at most single excitations (photons). By conducting the
analysis on a general level we allow for an arbitrary initial state of the QEF
(which can be for instance: the vacuum, the ground state in a cavity, or the
squeezed state). We derive a Volterra-type equation which governs the time
evolution of the amplitude of the excited state. The two-point function of the
initial state of the QEF, integrated with a combination of atomic
wavefunctions, forms the kernel of this equation.
