  We present a non-relativistic theory of quantum radiation generated by
shaking a two-level atom in vacuum. Such radiation has the same origin of
photon emission in dynamical Casimir effect. By performing a time-dependent
"dressing" transformation to the Hamiltonian, we derive an interaction term
that governs the radiation. In particular, we show that photon pairs can be
generated, not only by shaking the position of the atom, but also by changing
the internal states of the atom. As applications of our theory, we calculate
the emission rate from an oscillating atom, and the multi-photon state
generated in a single-photon scattering process.
