Mass-selected Cluster Ions Research Articles

Photodissociation of rare gas cluster ions: Ar+3

The photodissociation spectrum of the mass selected cluster ion Ar+3, produced by electron impact ionization of a supersonic cluster beam, has been measured for the first time. Measurements over the wavelength range 539 to 620 nm show the cross section to rise and peak near 545 nm and then decrease steadily throughout the longer wavelength range; the magnitude of the cross section is found to be smaller than the corresponding 2Σ+u→2Σ+g transition of Ar+2 in agreement with some theoretical predictions. Interestingly, Ar+ is the only detected photoproduct.

Ultraviolet photoelectron spectroscopy of semiconductor clusters: Silicon and germanium

Ultraviolet photoelectron spectra (UPS) are reported here for semiconductor clusters prepared in a supersonic beam. Using a new magnetically focused time-of-flight photoelectron spectrometer, UPS spectra were obtained for mass-selected negative cluster ions of silicon and germanium in the 3-12-atom size range. An ArF excimer laser (6.4 eV) was used for photodetachment, enabling the first 3–4 eV of the valence band structure of the clusters to be probed. With few exceptions, the UPS data for corresponding clusters of the two semiconductors were remarkably similar. The spectra suggest that clusters 4, 6, 7, and 10 of silicon and 4, 6 and 7 of germanium are closed-shell species with band gaps of 1 to 1.5 eV.

Covalent Group IV Atomic Clusters

Atomic clusters containing from two to several hundred atoms offer the possibility of studying the transition from molecules to crystalline solids. The covalent group IV elements carbon, silicon, and germanium are now being examined with this long-range objective. These elements are particularly interesting because of the very different character of their crystalline solids and because they are intermediate between metals and insulators in the nature of their bonding. Small mass-selected atom cluster ions are formed by pulsed laser techniques and identified by time-of-flight methods. Laser photoexcitation is used to study the relative stability of these clusters and their modes of fragmentation. These modes for C(n)(+) clusters, which tend to fragment with a characteristic loss of a neutral C(3), are found to be different from the modes for Si(n)(+) and Ge(n)(+) clusters, which tend to fragment to "magic" clusters such as Si(4)(+), Si(6)(+) and Si(10)(+). These experimental results can be accounted for by recent theoretical calculations of the ground-state structure and stability of small silicon and carbon clusters. Several theoretical approaches give consistent results, showing that small silicon clusters are compact and different from small fragments of the bulk crystal. Calculations show that carbon clusters change from linear structures toward cyclic structures as the cluster size increases, but with significant odd-even differences.

Ion molecule reactions of carbon cluster ions with D2 and O2

The ion/molecule chemistry of laser-generated carbon cluster ions (C+n; n=3–19) has been studied using Fourier transform mass spectrometry. The ion/molecule reactions of mass-selected carbon cluster ions with D2 and O2 have been studied and reaction rates measured. Reactions of the primary product ions with D2 and O2 were also studied. The change in reactivity observed as a function of cluster size suggests a structural change from linear to monocyclic rings in the cluster ions between n=9 and 10. Evidence for the presence of two structural isomers for the C+7 cluster ion has also been observed and has been attributed to the existence of both a linear and cyclic form of the ion. MNDO calculations have also been used to obtain structural and thermodynamic information on possible reactant and product ions in an attempt to explain the differences observed in the ion/molecule reactions. The results from collision induced dissociation studies of the carbon cluster ions are also discussed.

Metal cluster ion photofragmentation

Photofragmentation studies are described for mass-selected transition metal cluster ions cooled in a supersonic expansion. Examples of the spectral utility of this technique are presented in the cases of Ni+2 and Nb+2 for which well resolved vibronic bands have been recorded by monitoring fragment yield as a function of dissociation laser wavelength. For larger, more complicated metal clusters the absorption spectrum is far too congested and perturbed for analysis, but photodissociation can reveal the thresholds and time scales of various fragmentation pathways. The order of the absorption event leading to dissociation is found from the extent of fragmentation as a function of laser fluence. Using this method, the one-photon dissociation threshold of Fe+2 is found to lie between 2.43 and 2.92 eV, which determines the binding energy of this transition metal dimer cation. Combined with previous ionization potential measurements this places the binding energy of the Fe2 neutral between 0.83 and 1.32 eV. Similarly, the threshold for dissociation of Fe+3 into Fe+2 +Fe was found to lie between 1.17 and 2.18 eV, placing the Fe3 neutral binding energy between 1.27 and 2.38 eV. For all transition metal clusters measured (Fex, Nix, and Nbx with x between 2 and 10) the primary one-photon fragmentation pathway was always found to be loss of a single metal atom from the cluster. Far more accurate measures of these dissociation thresholds should be possible with more extensive measurements of cold cluster ion photodissociation as a function of wavelength. Arguments are presented to the effect that these dissociation thresholds should generally provide valid measures of the true binding energies of transition metal clusters.

Neutralization of mass-selected cluster ions by charge transfer reactions

Mass-separated beams of clusters with a single, well-defined number of atoms or molecules are indispensable for studies of cluster properties. This paper presents a technique with the potential to generate such a beam which contains neutral clusters of one size only. For this purpose mass-selected cluster ions have been neutralized by charge transfer reactions in metal and sulphur vapors. Relative charge exchange cross sections were measured for different cluster sizes. They reveal pronounced variations of the electronic structure of these particles. Thus, the study of charge transfer processes is introduced as a method to probe the positions of electronic energy levels in neutral clusters.

Methods for generating neutral, mass-selected cluster beams

Methods for generating a neutral, mass-selected cluster beam are discussed. They involve charge exchange of mass-selected cluster ions, velocity selection of neutral clusters produced with the gas aggregation technique, and deflection of clusters in inhomogeneous electric fields. Experiments with the goal to test these methods have been started on Na n and S n clusters. First experimental evidence is given that mass-selected cluster ions can be neutralized by charge transfer in an alkali vapor.