Inelastic Electron Tunneling (IET)
Definition and meaning of Inelastic Electron Tunneling (IET) in chemistry.
Inelastic electron tunneling is a quantum process where an electron passes through a thin barrier. During this trip, the electron loses a tiny, specific package of energy to a nearby molecule. This lost energy causes the molecule to vibrate, which helps chemists identify the substance.
In more detail
In classical physics, an electron cannot cross a solid barrier of insulating material. However, quantum mechanics shows that small particles can tunnel through these barriers like ghosts. In elastic tunneling, the electron crosses the barrier with no loss of energy.
In the inelastic version, the electron gives up energy to excite a molecular vibration. The amount of energy lost matches the exact vibrational level of the trapped molecule. Chemists measure this energy loss using a special technique called inelastic electron tunneling spectroscopy.
They apply a voltage across the barrier and measure how the current changes. A graph of these changes shows distinct peaks at voltages that match the vibrational energies. This method is highly sensitive and lets scientists identify a single layer of molecules.
It works much better than regular infrared or Raman spectroscopy for molecules trapped in narrow junctions. Students often get confused and think this process changes the chemical bonds of the molecule. In reality, the molecule only vibrates temporarily and does not break apart or change its identity.
Key facts
| Field | Physical Chemistry |
|---|---|
| Full Name | Inelastic Electron Tunneling |
| Measurement Method | Inelastic electron tunneling spectroscopy (IETS) |
| Typical Energy Scale | 10 to 500 millielectronvolts (meV) |
| Primary Output | Second derivative of current with respect to voltage |
| Application Area | Surface chemistry and molecular electronics |
Consider a thin layer of acetic acid trapped inside a metal-oxide-metal junction. When we run a tunneling current through this junction, we see a clear signal peak. This peak appears at a voltage that matches the vibration of the carbon-hydrogen bond in the acid. The location of the peak confirms that acetic acid is present in the junction. It also tells scientists how the molecule is oriented on the metal surface.
Frequently asked questions
How does IET differ from elastic tunneling?
In elastic tunneling the electron loses no energy as it crosses. In inelastic tunneling the electron loses energy to make a molecule vibrate.
Why is IET useful in chemistry?
It can detect extremely small amounts of molecules trapped on metal surfaces. This sensitivity is much higher than standard optical methods.
Does inelastic tunneling damage the molecule?
No it does not. The electron only transfers a tiny amount of energy to cause normal vibration without breaking chemical bonds.