Anthony J. Stace

Reactivity‐structure correlations in ion clusters: A study of the unimolecular fragmentation patterns of argon ion clusters, Ar+n, for n in the range 30–200

A high resolution double focusing mass spectrometer in association with a pulsed supersonic nozzle has been used to study the unimolecular fragmentation of Ar+n clusters for n in the range 30–200. All ion clusters within this range are observed to undergo the reaction Ar+n→Ar+n−1+Ar; with some ions also exhibiting further fragmentation involving the loss of two and three argon atoms. A systematic study of the above reaction as a function of n reveals that the relative intensities of the fragment ions fluctuate considerably. In many cases, these fluctuations coincide with depleted or enhanced ion intensities in the normal ion cluster mass spectrum. Through the use of a model based on unimolecular reaction rate theory (RRKM), it has been possible to reproduce many of the features observed in both the fragmentation spectra and the normal mass spectrum. Of the two principle variables involved in the model, reaction symmetry number and dissociation or binding energy, we have been able to show that only the bin...

A classical trajectory study of collisional energy transfer in thermal unimolecular reactions

Classical trajectory calculations have been carried out to study energy transfer in atom–triatom collisions. Collisions between O3 and He, Ar and Xe have been studied at energies corresponding to the temperature range 300–2500 K and between H2O and Ar in the range 2500–10 000 K. The results allow a comparison between the multistep and strong collision models of unimolecular decay. For both O3 and H2O the multistep model gives good agreement with experimental results and the energy transfer characteristics of He, etc. conform to those expected for an inefficient heat‐bath gas. Rotational energy transfer is found to be more efficient than vibrational and energy transfer in general is sensitive to the R−6 term in the atom–triatom potential.

The photofragmentation of Ar+3

The photofragment kinetic energy spectrum of Ar+3 has been recorded in a crossed‐beam apparatus at 532 nm; the only photofragment product observed is Ar+. For the purposes of analysis, a spectrum for the photofragmentation of Ar+2 was recorded under similar experimental conditions. In each case, the ions were prepared by the electron impact ionization of a neutral argon cluster beam. The Ar+3 spectrum consists of two quite distinct features, a high‐energy component which closely resembles the result observed for Ar+2, and a second, low‐energy feature, which is peculiar to Ar+3 alone. The two high‐energy wings appear to arise from a very rapid dissociation process where approximately 70% of the excess energy appears as Ar+ kinetic energy. A computer simulation of this region of the spectrum gives an anisotropy parameter, β, of 1.1±0.2. The low‐energy, component to the spectrum arises from a two‐step dissociation process, in which a weakly bound atom carries away a relatively large fraction of the available...

Molecular dynamics and chemical reactivity

A computer simulation has been obtained of the atom recombination reaction in which the recombination energy is removed by a third-body. The equations of classical dynamics have been solved for two iodine atoms and six inert gas atoms (He, Ar or Xe) confined to a spherical vessel by a wall potential which is a spatial average of a spherical shell of inert gas atoms. Reactions at a given temperature and concentration are simulated by varying the initial momenta of the atoms and the volume of the sphere. A computer run of 100 trajectories for each physical situation gives the average time for recombination, and from this a macroscopic rate constant has been calculated that agrees well with the experimental result. The model reproduces all the characteristic kinetic mechanisms that have traditionally been used to interpret atom recombination. However, at high inert gas concentrations the steady-state approximation is shown to fail as many of the important intermediate reactions do not reach equilibrium. In t...

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

A double‐focusing mass spectrometer in conjunction with a cluster beam source has been used to measure the average kinetic energy released following the unimolecular and collision‐induced fragmentation (CID) of argon cluster ions. Measurements on unimolecular decay have been made for clusters in the range Ar+5–Ar+60, and for the CID studies the range was Ar+2–Ar+30. Within the observation time window, the kinetic energy release results for the loss of a single argon atom via unimolecular decay are consistent with internal energy being partitioned statistically. Three separate CID routes are identified: (i) loss of one Ar atom; (ii) rapid ( 10−7 s. It is proposed that the CID of small cluster ions proceeds via electronic excitation; but that as the clusters increase in size (n>4) vibrational excitation predominates. A simple spectator model of collisional excitation accounts for the ...

Observations of Coulomb explosion in doubly charged atomic and molecular clusters

Coulomb explosion has been promoted in a range of doubly charged atomic and molecular clusters. In these new experiments, mass selected clusters of Ar2+n, (CO2)2+n, (H2O)2+n, (H2O)nH2+2, (CH3CN)nH2+2, and (C6H6)2+n have been subjected to collisional activation with a background gas. For species close to the Coulomb cutoff, each collision removes sufficient atoms or molecules (approximately six) as to render the clusters unstable. As a result, charge separation occurs and part (≂30%) of the Coulomb repulsion energy is released in the form of center of mass kinetic energy in the fragments. The remaining Coulomb energy appears as internal excitation in the fragments and subsequently leads to extensive evaporation. It is shown that the latter process is continuing even 10−6 s after Coulomb explosion. All the molecular systems studied show evidence of asymmetric charge separation, with some singly charged fragments containing up to 65% of the initial cluster mass. A detailed quantitative analysis of the result...

Electrostatic analysis of the interactions between charged particles of dielectric materials

An understanding of the electrostatic interactions that exist between charged particles of dielectric materials has applications that span much of chemistry, physics, biology, and engineering. Areas of interest include cloud formation, ink-jet printing, and the stability of emulsions. A general solution to the problem of calculating electrostatic interactions between charged dielectric particles is presented. The solution converges very rapidly for low values of the dielectric constant and is stable up to the point where particles touch. Through applications to unspecified particles with a range of size and charge ratios, the model shows that there exist distinct regions of dielectric space where particles with the same sign of charge are strongly attracted to one another.

On the origin of metastable decay in Ar+2

Metastable Ar+2 (Ar+2 →Ar++Ar) has been observed in a double‐focusing mass spectrometer from ions created by 70 eV electron bombardment of an Ar cluster beam. New ground and excited state potential energy curves have been calculated for Ar+2, and these have been used to show that metastability is due to radiative decay from the II(1/2)u state of the ion. It is shown that vertical (FC) ionization from neutral Ar2, with a vibrational temperature of approximately 30 K, results in a significant fraction of the ions occupying the II(1/2)u state. Detailed pressure dependent measurements show that collision‐induced dissociation does not contribute to the observed Ar+ signal. The mean kinetic energy released to the Ar+ has been measured as 44 cm−1 in the center‐of‐mass frame, and calculations show that this value is consistent with the proposed mechanism.

Metal ions in hydrogen bonded solvents: a gas phase perspective

Recent experiments and complementary ab initio calculations have focused attention on studying multiply-charged metal–solvent [M·Sn]q+ complexes in the gas phase. Although the preparation and study of such complexes presents a considerable experimental challenge, techniques capable of yielding quantitative signals are beginning to emerge. When the solvent (S) consists of a molecule capable of forming hydrogen bonds, e.g. H2O or CH3OH, evidence is frequently found of an extended solvent structure, which theory attributes to the formation of charge-enhanced hydrogen bonds. In some instances, these hydrogen bonds are formed in preference to completing the shell of solvent molecules, which should surround the central ion. As a consequence, gas phase and condensed phase experiments would appear to yield different solvation numbers; however, it is possible to rationalize the two data sets. Experiments on individual [M·Sn]q+ complexes could lead to a molecular picture of the mechanism for hydrolysis; a process which could be viewed as the chemical consequences of hydrogen bonding.

Unexpected stability of [Cu⋅Ar]2+, [Ag⋅Ar]2+, [Au⋅Ar]2+, and their larger clusters

Experimental observations following the ionization of neutral group 11 metal/argon complexes have revealed the presence of doubly charged ions of the form [M . Ar-n](2+) for n in the range 1-6. Of particular interest are two features of the results. First, the unexpected stability of the dimer ions, [M . Ar](2+), since similar species involving a molecule rather than a rare gas atom are often unstable with respect to charge transfer. Ab initio calculations show the dimers owe their stability to a combination of a strong electrostatic interaction and the high ionization energy of argon. A second feature to the results is the high relative intensities of the [M . Ar-4](2+) and [M . Ar-6](2+) ions. Calculations show these complexes to consist of square-planar D-4h structures, with the additional two atoms in [M . Ar-6](2+) occupying axial sites, which are Jahn-Teller distorted. The calculated relative binding energies support the preferential stability of these two structures