Coulomb fission in multiply charged molecular clusters: Experiment and theory

Abstract

A series of three multiply charged molecular clusters, (C6H6)n z+ (benzene), (CH3CN)n z+ (acetonitrile), and (C4H8O)n z+ (tetrahydrofuran), where the charge z is either 3 or 4, have been studied for the purpose of identifying patterns of behaviour close to the charge instability limit.Experiments show that on a time scale of ~10 -4 s, ions close to the limit undergo Coulomb fission where all of the observed pathways exhibit considerable asymmetry in the sizes of the charged fragments, and are associated with kinetic (ejection) energies of between 1.4 and 2.2 eV. Accurate kinetic energies have been determined through a computer simulation of peak profiles recorded in the experiments and the results modelled using a theory formulated to describe how charged particles of dielectric materials interact with one another (Bichoutskaia et al. J. Chem. Phys. 2010, 133, 024105). The calculated electrostatic interaction energy between separating fragments gives an accurate account for the measured kinetic energies and also supports the conclusion that +4 ions fragment into +3 and +1 products as opposed to the alternative of two +2 fragments. This close match between theory and experiment supports the assumption that a significant fraction of excess charge resides on the surfaces of the fragment ions. It is proposed that the high degree of asymmetry seen in the fragmentation patterns of the multiply charged clusters is due, in part, to limits imposed by the time window during which observations are made.2

Interaction between particles with inhomogeneous surface charge distributions: Revisiting the Coulomb fission of dication molecular clusters

“…As discussed in earlier publications [16][17][18] , observations on the fragmentation patterns of multiply charged molecular clusters have been undertaken on an apparatus that combines a high resolution reversed geometry mass spectrometer (VG Analytical ZAB-E) with a pulsed supersonic cluster source. This is the author's peer reviewed, accepted manuscript.…”

“…Figure 2 gives an example of a kinetic energy spectrum recorded following the Coulomb fission of [(H 2 O) 37 ] 2+ , and as noted above, the key measurement in these experiments is the kinetic energy released as a consequence of Coulomb repulsion between the two like-charged fragments as they separate. Due to a combination of high kinetic energy release and comparatively light mass, the peak profiles recorded from the fragmentation of molecular dications show the presence of a sequence of pronounced dish-shaped peak profiles 16,17 . These arise from the preferential transmission of fragment ions that are either strongly forward or backward scattered with respect to the laboratory frame of reference.…”

“…To improve the accuracy of energy release measurements, a method for calculating peak profiles proposed by Beynon and co-workers 41,42 has been adopted in the form of a Monte Carlo simulation 17 . From random values for the kinetic energy release, centre-of-mass velocities for the fragments are calculated on the assumption that, in the centre of mass frame, the scattering of ions is equally probable in all direction.…”

“…In this work we have addressed the latter topic with examples taken from the fragmentation of dication molecular clusters. As has been discussed in earlier publications on the fragmentation of dication clusters, [16][17][18] there are other contributions to the energy barrier experienced by a dication at the onset to fragmentation, and these will come from the breaking of physical and/or This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.…”

“…Identifying the point at which highly charged, finite collections of atoms and molecules become unstable has been the subject of numerous experimental and theoretical studies, [4][5][6][7][8][9][10][11][12][13][14] but it is only recently that detailed patterns of charge separation close to the Rayleigh instability limit have been established. 15,16 Molecular systems for which specific fragmentation patterns have been mapped include: (H 2 O) 2+ n , (NH 3 ) 2+ n , (CH 3 CN) 2+ n , (C 5 H 5 N) 2+ n , (C 6 H 6 ) 2+ n ; 16 (C 6 H 6 ) z+ n , (CH 3 CN) z+ n and (C 4 H 8 O) z+ n with z = 3 and z = 4; 17 and (NH 3 ) z+ n clusters with z ≤ 8, 18 . Precise quantitative data are available for a range of doubly charged clusters, where Coulomb fission has been shown to result in asymmetric fragmentation and the formation of two singly charged clusters.…”