A simple model for laser-electrode interaction and its role in photo-assisted electron transport processes in molecular interfaces

A simple model to assess the relative importance in photo-assisted conductance of laser-induced excitation processes in metal electrodes is presented. We consider both one and two electrodes subject to the combined effect of a dc bias and a time-dependent field using a model for the photo-induced dynamic electron distribution function. Floquet formalism is used to analyse the multiphoton processes inducing electron tunnelling through the surface. Under the assumption of low laser intensity, the relative importance of the direct current and of the one resulting from photon absorption are carefully examined by varying the characteristics of the external fields (applied voltage for the static field, intensity and frequency for the laser). For the one-electrode case, our approach renders results that are consistent with other available treatments for photofield emission current. Under certa in conditions, that may be reproduced in current experiments, we observe a great sensitivity of the single-photon current with respect to the laser intensity and frequency, advocating for the use of a laser modified electron distribution in the metal instead of the commonly used equilibrium Fermi-Dirac distribution. However, for the case of a vacuum junction with two metal electrodes, we find that these transient effects can be safely neglected for the transport problem, under current experimental limitations. This seems to justify commonly used models that include only the molecule-laser interaction in the calculation of the conductance but leave the possibility open for measurable consequences in situations where there is competition between through-space and through-molecule transport.