Inelastic Electron Tunneling (IET)
Definition and meaning of Inelastic Electron Tunneling (IET) in chemistry.
Inelastic electron tunneling (IET) is a quantum process in which an electron tunneling through a thin insulating barrier loses a small, discrete amount of energy by exciting a vibrational mode of a molecule sitting within the barrier. It contrasts with elastic tunneling, where the electron passes through with no energy loss.
In more detail
Because the lost energy matches a specific vibrational quantum of the molecule, plotting the second derivative of tunneling current with respect to voltage reveals peaks at voltages corresponding to those vibrational energies. This is the basis of inelastic electron tunneling spectroscopy (IETS), which probes the vibrational fingerprint of a single molecule or thin molecular layer sandwiched between two metal electrodes. IETS offers different selection rules and higher sensitivity than infrared or Raman spectroscopy for molecules confined to such junctions, making it valuable in surface chemistry and molecular electronics.
Key facts
| Field | Physical Chemistry |
|---|---|
| Full name | Inelastic Electron Tunneling |
| Related technique | Inelastic Electron Tunneling Spectroscopy (IETS) |
| Typical energy scale | A few meV to ~500 meV, matching molecular vibrational quanta |
In a metal-oxide-metal junction containing a monolayer of acetic acid, IET produces a distinct peak in the IETS spectrum at the voltage matching the molecule's C-H stretching vibration, confirming the molecule's presence and orientation within the junction.
Frequently asked questions
How does IET differ from elastic tunneling?
In elastic tunneling the electron crosses the barrier with no energy loss. In IET, the electron gives up a quantum of energy to excite a vibrational mode of a molecule in the junction, so it emerges with slightly less energy than it started with.
Why is IET useful in chemistry?
It underlies IETS, a vibrational spectroscopy technique that identifies molecules and their bonding geometry in tunnel junctions, complementing infrared and Raman spectroscopy, especially for molecules that are otherwise hard to probe optically.