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 ͑IETS͒ of two sample molecules on generic model substrates as typically accessible within a scanning tunneling microscopy ͑STM͒ 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.

Intrusive STM imaging

“…50 Assuming that δE i E i , the local density of states (LDOS) ρ(E,r) of the entire system can be approximated by a Gaussian broadening γ i similar to the discrete LDOS of the isolated cluster: This approach allows us to efficiently describe both resonant 46 Note that computing the precise form of the LDOS spectrum would require a radical change of focus to consider the energy E as the independent variable instead of finding the resonant energies E i (eigenvalues) of the system. Such approach has been used by several authors to compute wave functions from Green's functions, 51,52 and should give a more accurate description of the contacts. Nevertheless, the choice of using atomic clusters allows us to more efficiently construct the molecular orbitals using the resonant states (computed once).…”