  We obtain exact results for the transport through a resonant level model
(noninteracting Anderson impurity model) for rectangular voltage bias as a
function of time. We study both the transient behavior after switching on the
tunneling at time t = 0 and the ensuing steady state behavior. Explicit
expressions are obtained for the ac-current in the linear response regime and
beyond for large voltage bias. Among other effects, we observe current ringing
and PAT (photon assisted tunneling) oscillations.
