  The transport properties of a simple model for a finite level structure (a
molecule or a dot) connected to metal electrodes in an alternating current
scanning tunneling microscope (AC-STM) configuration is studied. The finite
level structure is assumed to have strong binding properties with the metallic
substrate, and the bias between the STM tip and the hybrid metal-molecule
interface has both an AC and a DC component. The finite frequency current
response and the zero frequency photo-assisted shot noise are computed using
the Keldysh technique, and examples for a single site molecule (a quantum dot)
and for a two-site molecule are examined. The model may be useful for the
interpretation of recent experiments using an AC-STM for the study of both
conducting and insulating surfaces, where the third harmonic component of the
current is measured. The zero frequency photo-assisted shot noise serves as a
useful diagnosis for analyzing the energy level structure of the molecule. The
present work motivates the need for further analysis of current fluctuations in
electronic molecular transport.
