  We consider the time evolution of the occupation probabilities for the 2s-2p
transition in a hydrogen atom interacting with an external field, V(t). A
two-state model and a dipole approximation are used. In the case of degenerate
energy levels an analytical solution of the time-dependent Shroedinger equation
for the probability amplitudes exists. The form of the solution allows one to
choose the ratio of the field amplitude to its frequency that leads to temporal
trapping of electrons in specific states. The analytic solution is valid when
the separation of the energy levels is small compared to the energy of the
interacting radiation.
