J. H. Beynon

Thermochemistry and energy partitioning in the charge separation reactions of doubly charged triatomic ions

Ion kinetic energy and appearance potential measurements have been used to study the unimolecular charge separation reactions of the triatomic ions CO22+, COS2+, CS22+, N2O2+, NO22+ ·, and SO22+. The experimentally determined metastable peak shapes have been compared with calculated peak shapes including calculations that determine both the average kinetic energy release and the distribution about this average. The product ion states have been assigned from the thermochemical data, and in all the reactions total spin and angular momentum are found to be conserved. There is evidence that the metastable peaks are associated with maxima on single potential surfaces rather than with a slow predissociation. For CO2++ and COS++ almost all the available energy is partitioned into translational energy of product separation.

ION KINETIC ENERGY SPECTROMETER

Abstract The development of ion kinetic energy spectrometry is used to illustrate how a new technique becomes broad ranging as new applications are found for it. Special emphasis is given to a description of the instrumentation and the philosophy underlying modifications made to it. Important sections of the paper discuss performance in terms of energy, mass and angular resolution and show that the requirements cannot always be optimized simultaneously. Useful scanning procedures and methods of ion detection and data acquisition are also assessed. The basic measurement of ion kinetic energy is used to study unimolecular, ion/molecule and ion/surface reactions. Peak shapes are analyzed for distributions of translational energy release and for energy change in collision processes that are not accompanied by fragmentation. The application of ion kinetic energy spectrometry to various problems in analysis are explored and likely future developments of the subject discussed.

Energy partitioning by mass spectrometry: Chloroalkanes and chloroalkenes

HCl elimination from vibrationally exicted C2–C4 chloroalkane and chloroalkene ions has been studied by ion kinetic energy spectrometry. The kinetic energy release during unimolecular decomposition was measured from the width of the metastable peak and used, together with other data, to infer the reaction mechanism. An average kinetic energy release of 438–614 meV was associated with 1,2‐elimination except in the chloroethane molecular ion. A much smaller kinetic energy release (∼ 30 meV) was associated with the 1,3‐elimination mechanism. In a few cases, composite metastable peaks were observed due to HCl elimination via two competitive reaction channels. In one such instance, involving the ion C4H7Cl+·, several different methods of generating the ion were used. This had a considerable effect on the relative weights of the two pathways but only a small effect upon the kinetic energy releases. The thermochemistry of many of the 1,2‐elimination reactions has been determined from ionization and appearance po...

Kinetic Energy Release in Unimolecular Ionic Reactions. Thermochemical Aspects

The kinetic energy (T) released in the unimolecular reactions of metastable gaseous ions has been compared with literature values obtained by several methods for the same reactions occurring in the ion source. The internal energy difference between the long‐lived and the short‐lived reactant ions has a relatively small effect upon the kinetic energy release when rearrangement and elimination reactions are studied, large kinetic energy releases being observed by both methods. Simple cleavages, which are typically accompanied by far smaller energy releases, show large relative variations when the time‐scale of the reaction is changed with the values measured in the field‐free region being much lower (e.g., 0.002 eV vs 0.18 eV in the case of H loss from propane). These and other results provide strong evidence that metastable reactions occur from ions whose average internal energies are only slightly in excess of the activation energy for the process in question. The consequences of this result for studies o...

The ion kinetic energy spectrum and the mass spectrum of argon

Abstract The technique of Ion Kinetic Energy Spectroscopy (I.K.E.S.) is described and results obtained with a sample of argon gas are discussed. When coupled with conventional mass spectroscopy, the technique enables the route to formation of the ions in the mass spectrum to be deduced. It can provide information on unimolecular or collision-induced decompositions of meta-stable ions. Its application to the study of the reactions of high-kinetic energy argon ions with nitrogen molecules is described.

The shapes of metastable peaks

Abstract A new precise method is presented for calculating the path of an ion through a radial electric-field. This has been applied to the calculation of the shapes of ion kinetic energy peaks due to the fragmentation of metastable ions in the field-free region in front of the electric sector of a mass spectrometer.

Kinetic Energy Release in the Dissociation of Some Simple Molecular Ions. Water and Hydrogen Sulfide

The technique of ion kinetic energy spectroscopy has been applied to a study of S+. formation from H2S+. and O+. formation from H2O+.. Unimolecular formation of S+. occurs by predissociation of the first excited state via the repulsive 4A2 state of H2S+.. Above the classical crossover region this reaction proceeds rapidly on the mass spectrometer time scale but tunneling through the barrier occurs slowly and gives rise to metastable ions which fragment with conversion of all the available potential energy to kinetic energy. Collisional excitation of ground state H2S+. yields excited ions which rapidly dissociate via the 4A2 repulsive surface to give substantially excited (v=2) H2. This reaction occurs with the partitioning of some 30% of the available energy into translational energy of the products. The heat of formation of S+., determined from the appearance potential, requires only slight correction for the excess energy term arising from the potential energy difference between the crossover region and...

Kinetics of unimolecular ionic reactions kinetic energy release in metastable ion fragmentations

Abstract The kinetic energy release for metastable ion fragmentations has been measured as a function of ion life-time by varying the ion accelerating voltage. These measurements have been used, assuming statistical partitioning of the non-fixed energy of the activated complex, to estimate the energy dependence of the unimolecular rate constant. By taking data at accelerating voltages as low as several hundred electron volts and by using two instruments of different geometries, ion life-times could be varied by a factor of 10. A decrease of less than 10% from an initial value of 75 meV (measured from the width of the peak at half maximum) was observed as the ion life-time was increased from 4 to 45 μs for H loss from the benzene molecular ion. No detectable variation in kinetic energy release from an initial value of 27 meV was observed between 4 and 37 μs for HCN loss from the benzonitrile molecular ion. Experiments in which the distribution of kinetic energies was determined confirmed these results. The energy dependence of the rate constant, estimated on the basis of statistical theory, was significantly greater than suggested by previous ion abundance studies for both the benzene and benzonitrile reactions. This discrepancy suggests that statistical theory may not apply to these reactions. This conclusion was supported by comparison of the experimental results with RRKM calculations which showed that for the theory to be applicable, the benzonitrile reaction cannot be occurring from the ground state. Reaction from a electronic state some 2 eV above the ground state explains the average value of the kinetic energy release but not its variation with ion life-time. It is concluded that this reaction cannot be described in conventional terms.

Collision-induced fragmentation of triatomic ions

Abstract A method for identifying energy states of the products of ion dissociations using ion kinetic energy spectrometry is described. The reactant triatomic ions at relative translational energies of 8 keV are allowed to undergo collision-induced dissociation and the positions of the peak maxima of the kinetic energy spectra are used to measure the excitation energy acquired by the ion during the vertical transition from the ground ionic state to the reactive surface. The kinetic energy released on fragmentation is determined from the peak shape and subtracted from the energy of the energized ion to fix the energy of the products. The large range of energies usually released suggests that reaction generally occurs on a repulsive surface. The following product states are deduced for the reactions listed: Except for the HCN+ reaction, the reacting states correlate with ground state diatomic ions and ground state diatomic molecules, although the neutral atoms and atomic ions are not necessarily formed in the ground electronic states. The assigned product states formed by collisional excitation correlate with reacting states which are spin allowed. Without exception, larger kinetic energy releases are associated with the process for which the positive charge resides with the diatomic fragments. The COS+ ion reacts via three pathways to give CS+ , S+ and CO+ as the charged fragments. The first two reactions are unexceptional. The last reaction is apparently due to COS+ in an electronically excited state reached prior to collision since the measured kinetic energy loss corresponds to an excitation energy which is 3.8 eV less than that required to reach the dissociation limit leading to CO+ + S in their ground electronic states.

Simulated MIKE spectra from a conventional double focusing mass spectrometer

Abstract By scanning the accelerating voltage ( V ) and the electric sector voltage ( E ) in a conventional Mattauch-Herzog geometry mass spectrometer such that E 2 / V is constant a simulated mass-analyzed ion kinetic energy (MIKE) spectrum is obtained. The spectrum obtained by the linked scan technique is similar to a MIKE spectrum except that individual peaks are narrow and do not provide information about the kinetic energy release. The sensitivity of this method of scanning compares well with the MIKE spectrum but artifacts due to the transmission of ions of adjacent masses are observed and these place some limitations on the linked scan method. Nevertheless, as an inexpensive alternative to construction of an instrument with reversed geometry, use of the present method in molecular structural analysis is foreseen.