Carrier wave effect in nonresonant inelastic scanning tunneling spectroscopy of molecules with delocalized frontier orbitals

Abstract

In this work we present a method for simulating the electron-vibration coupling in adsorbate molecules using a tight-binding formalism for the electronic degrees of freedom and a real-space description of the molecular vibrations. First, we derive a formalism which is very transparent in recognizing the effect of a local vibrational perturbation on the local electronic structure at any location. Second, we apply the method to nonresonant inelastic electron-tunneling spectroscopy (IETS) of two sample molecules on generic model substrates as typically accessible within a scanning tunneling microscopy (STM) experiment. The foremost observation is that the intensity of the STM-IETS signal is not necessarily strongest in the spatial vicinity of the vibrational perturbation. Instead, it is dependent on the delocalization of the molecular orbitals, which may lead to a strong intensity quite far from the vibration. Since this is an effect of the electronic structure and not related to the transfer of the vibrational motion itself, the molecular orbitals can be treated as "carrier waves" of the signal from a vibrational perturbation. This effect may be important in determining the orientation of an adsorbed molecule on a substrate surface or in manipulating adsorbed molecules with an STM tip. © 2010 The American Physical Society.