  We analyze resonant tunneling subject to a non-adiabatic time-dependent
bias-voltage through an asymmetric single molecular quantum dot with coupling
between the electronic and vibrational degrees of freedom using a {\em
Tien-Gordon-type} rate equation. Our results clearly exhibit the appearance of
photon-assisted satellites in the current-voltage characteristics and the
elimination of hot-phonon-induced negative differential conductance with
increasing ac driving amplitude for an asymmetric system. This can be ascribed
to an {\em ac-induced suppression} of unequilibrated (hot) phonons in an
asymmetric system.
