Rabi oscillations and few-level approximations in time-dependent density functional theory.

The resonant interaction of laser light with atoms is analyzed from the time-dependent density functional theory perspective using a model helium atom which can be solved exactly. It is found that in the exact exchange approximation the time-dependent dipole shows Rabi-type oscillations of its amplitude. However, the time-dependent density itself is not well described. These seemingly contradictory findings are analyzed. The Rabi-type oscillations are found to be essentially of classical origin. The incompatibility of time-dependent density functional theory with few-level approximations for the description of resonant dynamics is discussed.