  We investigate the analytical solution of a two-level system subject to a
monochromatical, linearly polarized external field that was published a couple
of years ago. In particular, we derive an explicit expression for the
quasienergy. Moreover, we calculate the time evolution of a typical two-level
system over a full period by evaluating series solutions of the confluent Heun
equation. This is possible without invoking the connection problem of this
equation since the complete time evolution of the system under consideration
can be reduced to that of the first quarter-period.
