Laser-assisted conductance of molecular wires: Two-photon contributions

We extend to second order in the field intensity a previously obtained first-order expression for the conductance of a tight-binding molecular wire, bridging two electrodes, illuminated by a weak laser. The higher-order term brings in corrections due to two-photon processes that, for low intensities and strong molecule-electrode contacts, are several orders of magnitude smaller than the first-order leading terms. For other ranges of parameters, the second-order correction may be significant mostly due to processes involving the absorption or emission of two photons.