Inelastic electron tunneling spectroscopy

Inelastic electron tunneling spectroscopy (IETS) is an experimental tool for studying the vibrations of molecular adsorbates on metal oxides.

The metal contacts are characterized by a constant density of states, filled up to the Fermi energy.

This is a second order effect in electron-vibration coupling, where a vibration is emitted and reabsorbed or vice versa.

Depending on the energetic parameters of the system, this correction may be negative and it may outweigh the positive contribution of the inelastic current, resulting in a dip in the IETS spectrum.

[7] Keeping the tip of a scanning tunneling microscope (STM) at fixed position over the surface and sweeping the bias voltage, one can record a I-V characteristic.

The second derivative gives information on vibrations of the adsorbate as in IETS, which is why this technique is commonly called STM-IETS.

In this case the role of the insulating oxide layer is played by the gap between the tip and the adsorbate.

The disadvantage of this method is that it is experimentally very challenging to create and identify a junction with exactly one molecule between the electrodes.

[17] The underlying physical mechanism that permits tunnelling electrons to excite atomic spin transitions has been studied by several authors.

A schematic drawing of the system under investigation, with two metal contacts (left and right reservoir), a molecule in (bridge electronic level) and a voltage applied between the two contacts. The wide band limit is assumed for both the contacts.
Left : Traveling electrons do not have sufficient energy to excite a vibration. Only elastic tunneling can take place.
Middle : When increasing the bias voltage beyond V=E/e (where e is the electronic charge), traveling electrons do have sufficient energy to excite a vibration with energy E. Inelastic tunneling can take place.
Right : Traveling electrons can also excite and subsequently reabsorb a vibration, which leads to second order elastic tunneling.
A change of slope in the current vs voltage leads to a step in the first derivative and to a peak in the second derivative of the current to the voltage.