Quantum tunnelling is where a particle is found outside a confining potential despite it having insufficient energy to cross the barrier classically.
The effect arises from the fact that a wavefunction does not fall abruptly to zero at the walls of a container (unless the potential is infinite), but decays exponentially inside the barrier. The result of this is that the wavefunction may be non-zero on the far side of the potential barrier and hence, by the Born interpretation of the wavefunction, there is some probability of finding the particle there.
It can be shown mathematically that the probability of tunnelling decreases exponentially with the width of the barrier and the square root of the mass of the particle. Thus tunnelling is very important for light particles such as electrons but is insignificant for heavier entities such as diatomic molecules because the probabilities involved are negligably small.
Designed by Peter Lewis, Undergraduate, School of Chemistry, University of Bristol.