  We theoretically examine the effect of the coupling of the transport
electrons to a vibrational mode of the molecule on the ac linear-response
conductance of molecular junctions. Representing the molecule by a single
electronic state, we find that at very low temperatures the frequency-dependent
conductance is mainly enhanced (suppressed) by the electron-vibration
interaction when the chemical potential is below (above) the energy of that
state. The vertex corrections of the electron-vibration interaction induce an
additional peak structure in the conductance, which can be observed by tuning
the tunnel couplings with the leads.
