Isomerisation: Don't forget the possibility of enhanced tunnelling! A simple two-state model for the on-resonance and near-resonance behaviour of enhanced tunnelling

Whether it be isomerisation or molecular alignment in an external field, resonance of energy levels can lead to greatly enhanced quantum tunnelling between potential energy wells. We present a very simple two state model that accounts for the on-resonance and near-resonance behaviour of the energy levels and, more importantly, of the dipole matrix elements involving these states. The predictions of this simple model are validated by more realistic calculations for a rigid rotor in an external field. We draw attention to the fact that an external field can result in such an energy coincidence, even if it does not exist in the unperturbed system. Such enhanced tunnelling may play an important role in uni-molecular reactions such as isomerisation and intra-molecular relaxation.