Measurements of kinetic energy release following the unimolecular and collision‐induced dissociation of argon cluster ions, Ar+n, for n in the range 2–60

Coulomb fission in multiply charged molecular clusters: Experiment and theory

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 the 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 the observed pathways exhibit considerable asymmetry in the sizes of the charged fragments and are all 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 [E. Bichoutskaia et al., J. Chem. Phys. 133, 024105 (2010)]. 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 the theory and experiment reinforces 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.

Photodissociation of medium-sized argon cluster cations in the visible region

Semiclassical methods for non-adiabatic dynamics simulations, based on a semiempirical diatomics-in-molecules model of intracluster interactions and the mean-field dynamical approach with the inclusion of quantum decoherence, have been used to study the photodissociation of argon cluster cations, Ar_N⁺ (N = 6–19), at E_phot = 2.35 eV.

Isomerization dynamics and thermodynamics of ionic argon clusters

The dynamics and thermodynamics of small Ar(n) (+) clusters, n=3, 6, and 9, are investigated using molecular dynamics (MD) and exchange Monte Carlo (MC) simulations. A diatomic-in-molecule Hamiltonian provides an accurate model for the electronic ground state potential energy surface. The microcanonical caloric curves calculated from MD and MC methods are shown to agree with each other, provided that the rigorous conservation of angular momentum is accounted for in the phase space density of the MC simulations. The previously proposed projective partition of the kinetic energy is used to assist MD simulations in interpreting the cluster dynamics in terms of inertial, internal, and external modes. The thermal behavior is correlated with the nature of the charged core in the cluster by computing a dedicated charge localization order parameter. We also perform systematic quenches to establish a connection with the various isomers. We find that the Ar(3) (+) cluster is very stable in its linear ground state geometry up to about 300 K, and then isomerizes to a T-shaped isomer in which a quasineutral atom lies around a charged dimer. In Ar(6) (+) and Ar(9) (+), the covalent trimer core is solvated by neutral atoms, and the weakly bound solvent shell melts at much lower energies, occasionally leading to a tetramer or pentamer core with weakly charged extremities. At high energies the core itself becomes metastable and the cluster transforms into Ar(2) (+) solvated by a fluid of neutral argon atoms.

Temperature Measurement from the Translational Kinetic Energy Release Distribution in Cluster Dissociation: A Theoretical Investigation

Unimolecular dissociation of neutral and charged argon clusters is theoretically investigated in the context of calorimetric measurements. The temperature of the product cluster is estimated from the distribution of the translational kinetic energy released (KER), assumed to have the form f(epsilon) approximately epsilon(alpha) exp(-epsilon/k(B)T). Phase space theory (PST) in its orbiting transition state (OTS) version is validated by comparing its predictions to the results of large-scale molecular dynamics simulations. The temperatures estimated from the KER distributions are seen to be generally lower than the actual microcanonical temperature computed from independent Monte Carlo simulations of the product cluster at thermal equilibrium. On the basis of these deviations, the various approximations leading from the rigorous PST/OTS treatment to the assumed exponential form are critically discussed. In the case of Ar(n)(+) clusters, the use of a quantum diatomic-in-molecules Hamiltonian constructed from recent ab initio calculations reveals some possible inadequacies of the 1/r(4) ion/dipole interaction at intermediate distances due to some residual charge transfer.

Activation of large ions in FT-ICR mass spectrometry

The advent of soft ionization techniques, notably electrospray and laser desorption ionization methods, has enabled the extension of mass spectrometric methods to large molecules and molecular complexes. This both greatly extends the applications of mass spectrometry and makes the activation and dissociation of complex ions an integral part of these applications. This review emphasizes the most promising methods for activation and dissociation of complex ions and presents this discussion in the context of general knowledge of reaction kinetics and dynamics largely established for small ions. We then introduce the characteristic differences associated with the higher number of internal degrees of freedom and high density of states associated with molecular complexity. This is reflected primarily in the kinetics of unimolecular dissociation of complex ions, particularly their slow decay and the higher energy content required to induce decomposition--the kinetic shift (KS). The longer trapping time of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) significantly reduces the KS, which presents several advantages over other methods for the investigation of dissociation of complex molecules. After discussing general principles of reaction dynamics related to collisional activation of ions, we describe conventional ways to achieve single- and multiple-collision activation in FT-ICR MS. Sustained off-resonance irradiation (SORI)--the simplest and most robust means of introducing the multiple collision activation process--is discussed in greatest detail. Details of implementation of this technique, required control of experimental parameters, limitations, and examples of very successful application of SORI-CID are described. The advantages of high mass resolving power and the ability to carry out several stages of mass selection and activation intrinsic to FT-ICR MS are demonstrated in several examples. Photodissociation of ions from small molecules can be effected using IR or UV/vis lasers and generally requires tuning lasers to specific wavelengths and/or utilizing high flux, multiphoton excitation to match energy levels in the ion. Photodissociation of complex ions is much easier to accomplish from the basic physics perspective. The quasi-continuum of vibrational states at room temperature makes it very easy to pump relatively large amounts of energy into complex ions and infrared multiphoton dissociation (IRMPD) is a powerful technique for characterizing large ions, particularly biologically relevant molecules. Since both SORI-CID and IRMPD are slow activation methods they have many common characteristics. They are also distinctly different because SORI-CID is intrinsically selective (only ions that have a cyclotron frequency close to the frequency of the excitation field are excited), whereas IRMPD is not (all ions that reside on the optical path of the laser are excited). There are advantages and disadvantages to each technique and in many applications they complement each other. In contrast with these slow activation methods, the less widely appreciated activation method of surface induced dissociation (SID) appears to offer unique advantages because excitation in SID occurs on a sub-picosecond time scale, instantaneously relative to the observation time of any mass spectrometer. Internal energy deposition is quite efficient and readily adjusted by altering the kinetic energy of the impacting ion. The shattering transition--instantaneous decomposition of the ion on the surface--observed at high collision energies enables access to dissociation channels that are not accessible using SORI-CID or IRMPD. Finally, we discuss some approaches for tailoring the surface to achieve particular aims in SID.

Tandem mass spectrometry: dissociation of ions by collisional activation

This review presents a brief historical introduction to the development of tandem mass spectrometry and its principal applications. It is placed in the context of the general principles underlying mass spectrometry, particularly the relationships between internal energy and fragmentation kinetics. The center-of-mass framework is presented as a convenient means of applying conservation of momentum to the energy transfer problem in tandem mass spectrometry as a means of deducing energy transfer in the collisional activation step and kinetic energy release as activated ions dissociate into fragment ions and neutrals. The principles of molecular beam methods are summarized and illustrative examples are given for which definitive information on reaction dynamics is available. The importance of scattering-very little appreciated in early discussions of tandem mass spectrometry-is shown to be the natural consequence of impulsive collisions, which appears to be a general mechanism for energy exchange in collisional activation. It is shown that the average energy transferred in single collisions is much less than the theoretical maximum given by the center-of-mass collision energy and the Massey criterion is presented as a simplistic rationale for understanding the essentially exponential decline in the energy transfer function above and below the relative velocity at which the probability for energy transfer is maximized. The issues of energy transfer in collisions of large molecular ions with low-mass neutrals are reviewed and a general description of energy transfer in multiple collisions is presented. It is shown that the center-of-mass and Massey criterion limitations are pragmatically overcome by multiple collision activation in ion traps. Surface-induced dissociation is presented as a viable alternative to multiple collision activation which is especially attractive for activation of large molecular ions. Finally, a few of the emerging dynamics principles governing energy transfer and dissociation of peptides are summarized. Copyright 2000 John Wiley & Sons, Ltd.

Tandem mass spectrometry: dissociation of ions by collisional activation

This review presents a brief historical introduction to the development of tandem mass spectrometry and its principal applications. It is placed in the context of the general principles underlying mass spectrometry, particularly the relationships between internal energy and fragmentation kinetics. The center-of-mass framework is presented as a convenient means of applying conservation of momentum to the energy transfer problem in tandem mass spectrometry as a means of deducing energy transfer in the collisional activation step and kinetic energy release as activated ions dissociate into fragment ions and neutrals. The principles of molecular beam methods are summarized and illustrative examples are given for which definitive information on reaction dynamics is available. The importance of scattering-very little appreciated in early discussions of tandem mass spectrometry-is shown to be the natural consequence of impulsive collisions, which appears to be a general mechanism for energy exchange in collisional activation. It is shown that the average energy transferred in single collisions is much less than the theoretical maximum given by the center-of-mass collision energy and the Massey criterion is presented as a simplistic rationale for understanding the essentially exponential decline in the energy transfer function above and below the relative velocity at which the probability for energy transfer is maximized. The issues of energy transfer in collisions of large molecular ions with low-mass neutrals are reviewed and a general description of energy transfer in multiple collisions is presented. It is shown that the center-of-mass and Massey criterion limitations are pragmatically overcome by multiple collision activation in ion traps. Surface-induced dissociation is presented as a viable alternative to multiple collision activation which is especially attractive for activation of large molecular ions. Finally, a few of the emerging dynamics principles governing energy transfer and dissociation of peptides are summarized. Copyright 2000 John Wiley & Sons, Ltd.