A molecule of ammonia, NH3, typically exists as an umbrella shape, a very stable structure that would normally be expected to require a large amount of energy to be inverted.
Researchers at MIT and Seoul National University have examined this phenomenon by using a very large electric field (up to 200,000,000 volts per meter) to suppress the simultaneous occupation of ammonia molecules in the normal and inverted states. This strong field is 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. However, quantum mechanically, the isolated molecule exists with equal probability in both valleys.
The molecule initially exists in either the normal or inverted structure, but it can tunnel spontaneously to the other structure. The amount of time required for that tunneling to occur is encoded in the energy level pattern. If the barrier between the two structures is high, the tunneling time is long. Under certain circumstances, such as application of a strong electric field, tunneling between the regular and inverted structures can be suppressed.
For many molecules, the barrier to tunneling is so high that tunneling would never happen during the lifespan of the universe. However, there are molecules like ammonia that can be induced to tunnel by careful tuning of the applied electric field. (SciTechDaily)