Multiply negatively charged aluminium clusters II.

Earlier experiments [Eur. Phys. J. D 43, 241 (2007)] investigating the production of aluminium cluster dianions have been extended to larger cluster sizes for the purpose of producing higher charge states. Polyanions are produced using the “electron-bath technique" which involves the exposure of monoanionic clusters to a bath of low-energy secondary electrons. In this contribution the production of Al_n ^3- is investigated as a function of cluster size. Relative trianion yields (number of trianions produced relative to the number of precursor anions) were observed to increase from a few percent up to 55% within ~30 cluster sizes. The observed appearance size (n = 103) is in reasonable agreement with the predictions of classical models.     

Multiply charged metal cluster anions

Formation and stability patterns of silver dianionic and gold trianionic clusters are investigated with Penning-trap experiments and a shell-correction method including shape deformations. The theoretical predictions pertaining to the appearance sizes and electronic shell effects are in remarkable agreement with the experiments. Decay of the multiply anionic clusters occurs predominantly by electron tunneling through a Coulomb barrier rather than via fission, leading to appearance sizes unrelated to those of multiply cationic clusters.     

Multiply charged titanium cluster anions: production and photodetachment

Titanium clusters are produced by laser vaporization of a metal wire in a helium gas pulse, stored in a Penning trap, size selected and transformed into multiply charged anions by electron attachment. Both doubly and triply charged titanium clusters are observed. For the first observation of photodetachment of electrons from metal cluster dianions, Ti ₅₅ ^2- clusters are selected and excited by a laser pulse, which leads to the emission of an excess electron: Ti ₅₅ ^2- → e⁻ + Ti ₅₅ ^- . This revised version was published online in August 2006 with corrections to the Cover Date.     

Coulomb instability of charged clusters

A model different from the Rayleigh model for Coulomb instability of charged metallic clusters is proposed. The two-component model of a metallic cluster in the quasi-classical approximation offers different critical charges depending on the type of charged particles. For small-sized parallelepiped clusters, the quantization of the electronic spectrum is taken into account. The critical sizes of Ag _N ^2? and Ag _N ^3? clusters are calculated in the framework of the proposed model. The results of calculations are in good agreement with experimental data. The Coulomb explosion of positively charged clusters Na _N ⁿ⁺ at 3≤n≤5 is explained qualitatively.     

Stability of Positively charged metallic clusters

Recent experimental results on the stability of positively charged metallic clusters are analyzed. The stability of multiply positively charged spherical Pb_n-clusters can be explained by assuming that metallic-like screening begins to occur in clusters with approximately 30 or more atoms and becomes bulk-like in clusters with approximately 300 atoms or more. Linearly shaped clusters Pb_n ⁺⁺ are stable forn6, but cease to exist for energetic reasons ifn13 atoms. Results for the stability of two-fold positively charged transition-metal clusters are also given.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Dissociation of doubly charged alkali metal clusters

We have studied the competition experimentally observed between fission and neutral atom evaporation, as dissociation channels of excited doubly charged sodium clusters, using the Density Functional Theory and the jellium model. The fission barrier has been obtained from an Extended Thomas-Fermi calculation including density gradient corrections to the kinetic energy of the electronic cloud. We discuss the influence of the coefficient of the density gradient term on the barrier height.     

Rayleigh instabilities in multiply charged sodium clusters

The stability of multiply charged sodium clusters Na(q+)(n) (q< or =10) produced in collisions between neutral clusters and multiply charged ions A(z+) ( z = 1 to 28) is experimentally investigated. Multiply charged clusters are formed within a large range of temperatures and fissilities. They are identified by means of a high-resolution reflectron-type time-of-flight mass spectrometer (m/deltam approximately 14 000). The maximum fissility of stable clusters is obtained for z = 28 and is X approximately 0.85+/-0.07, slightly below the Rayleigh limit (X = 1). It is mainly limited by the initial cluster temperature (T approximately 100 K).