Coulomb Fission in Multiply-Charged Ammonia Clusters: Accurate Measurements of the Rayleigh Instability Limit from Fragmentation Patterns

Christopher Harris,Anthony J Stace

doi:10.1021/acs.jpca.8b00815

Christopher Harris, Anthony J Stace

Open Access

https://doi.org/10.1021/acs.jpca.8b00815

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Abstract

A series of experiments have been undertaken on the fragmentation of multiply charged ammonia clusters, (NH3) nz+, where z ≤ 8 and n ≤ 850, to establish Rayleigh instability limits, whereby clusters at certain critical sizes become unstable due to Coulomb repulsion between the resident charges. Experimental results on size-selected clusters are found to be in excellent agreement with theoretical predictions of Rayleigh instability limits at all values of the charge. Electrostatic theory has been used to help identify fragmentation patterns on the assumption that the clusters separate into two dielectric spheres, and the predicted Coulomb repulsion energies used to establish pathways and the sizes of cluster fragments. 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. For clusters carrying a charge ≤+4, the results show that fragmentation proceeds via the loss of small, singly charged clusters. When clusters carry a charge of +5 or more, the experimental observations suggest a marked switch in behavior. Although the laboratory measurements equate to fragmentation via the loss of a large dication cluster, electrostatic theory supports an interpretation that involves the sequential loss of two smaller, singly charged clusters possibly accompanied by the extensive evaporation of neutral molecules. It is suggested that this change in fragmentation pattern is driven by the channelling of Coulomb repulsion energy into intermolecular modes within these larger clusters. Overall, the results appear to support the ion evaporation model that is frequently used to interpret electrospray experiments.

Highlights

The first quantitative interpretation concerned with the stability of multiply charged clusters and droplets was presented in 1882 by Lord Rayleigh.1 Rayleigh proposed that, as a consequence of electrostatic repulsion, a charged spherical mass of liquid could move from being in a state of stable equilibrium to becoming unstable as the magnitude of the charge is increased
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
Attempts to record data on the smaller of the two fragments were hampered by two factors: (i) the severe instrumental discrimination light ions with high kinetic energies can experience; and (ii) an overlap with peaks arising from the loss of neutral molecules, which has previously been shown to compete with Coulomb fission

Summary

Introduction

The first quantitative interpretation concerned with the stability of multiply charged clusters and droplets was presented in 1882 by Lord Rayleigh. Rayleigh proposed that, as a consequence of electrostatic repulsion, a charged spherical mass of liquid could move from being in a state of stable equilibrium to becoming unstable as the magnitude of the charge is increased. Coulomb fission has been found to occur through the metastable or unimolecular decay of mass-selected dication and trications molecular clusters.. Coulomb fission has been found to occur through the metastable or unimolecular decay of mass-selected dication and trications molecular clusters.11-13 These observations have shown that the decay process can be both delayed (~ 10-4 s) and very asymmetric; a typical example being (NH3)52H22+ (NH3)40H+ + (NH3)12H+,11-13 and it has been possible to interpret these experiments in terms of a two-body. As part of this process, information has been recorded on the fragmentation patterns of multiply charged clusters, where as part of the analysis, detailed electrostatic calculations have been undertaken to help interpret the data regarding the degree of asymmetry exhibited by the fragments both in terms of size and charge. The ion evaporation model (IEM) is thought to proceed via the ejection of small solvent ions, and may be more appropriate under circumstances where residue ions are found to carry just a fraction of the initial charge generated during ESI.

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