Polaron nature of critical size of ammonia cluster

A consistent polaron model is shown to result in a critical size of a cluster made of polar molecules. Over this size the existence of a bound state of an excess electron inside the cluster is possible. In the case of an ammonia cluster composed of n molecules the theory predicts an appearance of a stable polaron state of excess electron at n = 35. This threshold value and photodetachment energies are in a good agreement with existing experimental data. The theory predicts also metastable states of the ammonia cluster anions for 21 ≤ n < 35.