External field control of charge transmission through single molecules: Switching effects and transient currents

Net electron transfer through a single molecule attached to nano electrodes can undergo a pronounced change by resonant optical excitation. Caused by the population of excited electronic states of the molecule new transmission channels are opened which alter the current-voltage characteristics of the junction. This recent suggestion is further investigated here by studying the transient behavior of the current if the external laser pulse excitation is switched-on and -off within a certain time-interval. The computations concentrate on the case of weak and intermediate molecule-lead coupling. They are carried out in using rate equations which govern the populations of the different electron-vibrational states of the molecule being in its neutral or charged state. The interrelation of the switching-time and the time of charging and discharge of the molecule as well as of the time of vibrational relaxation is demonstrated by working in a switching-time range of some hundreds fs up to about 50 ps. (C) 2011 Elsevier B.V. All rights reserved.