Coulomb fission in multiply-charged ammonia clusters: accurate measurements of the Rayleigh instability limit from fragmentation patterns

A series of experiments have been undertaken on the fragmentation of multiply charged ammonia clusters, (NH3)nz+, where z 8 and n 800, to establish Rayleigh instability limits, whereby clusters at certain critical sizes become unstable due to Coulomb repulsion between the resident charges. The experimental results are in excellent agreement with theoretical predictions of critical size. Electrostatic theory has been used to help identify fragmentation patterns on the assumption that the clusters separate into two dielectric spheres. The predicted Coulomb repulsion energy is then used to match individual peak profiles recorded in the experiments in order to establish fragment size and the amount of charge they carry. The results show that fragmentation is very asymmetric in terms of both the numbers of molecules involved and the amount of charge each fragment accommodates. It is suggested that the results support the ion evaporation model that is frequently used to interpret electrospray experiments