A molecule of ammonia, NH3, typically exists as an umbrella shape, with three hydrogen atoms fanned out in a nonplanar arrangement around a central nitrogen atom.

This umbrella structure is very stable and would normally be expected to require a large amount of energy to be inverted.

However, a quantum mechanical phenomenon called tunneling allows ammonia and other molecules to simultaneously inhabit geometric structures that are separated by a prohibitively high energy barrier. by using a very large electric field to suppress the simultaneous occupation of ammonia molecules in the normal and inverted states.

The experiments, performed at Seoul National University, were enabled by the researchers’ new method for applying a very large electric field (up to 200,000,000 volts per meter) to a sample sandwiched between two electrodes.

This assembly is only a few hundred nanometers thick, and the electric field applied to it generates forces nearly as strong as the interactions between adjacent molecules.

We can apply these huge fields, which are almost the same magnitude as the fields that two molecules experience when they approach each other.

In the case of ammonia, the first valley is the low energy, stable umbrella state. For the molecule to reach the other valley the inverted state, which has exactly the same low energy classically it would need to ascend into a very high energy state.

Under quantum mechanics, the possible states of a molecule. The molecule initially exists in either the normal or inverted structure, but it can tunnel spontaneously to the other structure.

For ammonia, exposure to a strong electric field lowers the energy of one structure and raises the energy of the other (inverted) structure. As a result, all of the ammonia molecules can be found in the lower energy state.

The researchers demonstrated this by creating a layered argon-ammonia-argon structure at 10 kelvins. Argon is an inert gas which is solid at 10 K, but the ammonia molecules can rotate freely in the argon solid. As the electric field is increased, the energy states of the ammonia molecules change in such a way that the probabilities of finding the molecules in the normal and inverted states become increasingly far apart, and tunneling can no longer occur.

This effect is completely reversible and nondestructiv, As the electric field is decreased, the ammonia molecules return to their normal state of being simultaneously in both wells.

However, there are molecules other than ammonia that can be induced to tunnel by careful tuning of the applied electric field. His colleagues are now working on exploiting this approach with some of those molecules.

The electric field, because it’s so large, is capable of acting on the same scale as the actual chemical interactions,” offering a powerful way of externally manipulating molecular dynamics.


The study was published in Mit

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