Some of the tunneling electrons can lose energy by exciting vibrations of the oxide or the adsorbate. These inelastic processes lead to a second tunneling path, which gives an additional current contribution to the tunneling current. Since the incident electron should have enough energy to excite this vibration, there is a minimum energy that is the onset of this (inelastic) process. This is shown in the middle figure, where the lower dashed line is a vibronic state. This minimum energy for the electron corresponds with a minimum bias voltage, which is the onset for the additional contribution. The inelastic contribution to the current is small compared to the elastic tunneling current (~0.1%) and is more clearly seen as a peak in the second derivative of the current to the bias voltage, as can be seen in the bottom figure.
There is however also an important correction to the elastic component of the tunneling current at the onset. This is a second order effect in electron-vibration coupling, where a vibration is emitted and reabsorbed or vice versa. This is shown in the upper figure on the right. 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. This is experimentally verified in both regular IETS and in STM-IETS and is also theoretically observed. Not only peaks and dips may be observed, but depending on the energetic parameters also derivative-like features may be observed, both experimentally and theoretically.
Nowadays molecular transport junctions have been produced with one single molecule between two electrodes, possibly with an additional, gate electrode near the molecule. The advantage of this method in comparison with STM-IETS is that there is contact between both electrodes and the adsorbate, whereas in STM-IETS there is always a tunneling 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.