J. Leygnier

Dynamics of unimolecular dissociation of sodium cluster ions

We investigate the unimolecular dissociation dynamics of energy‐rich sodium cluster ions, Na+n (5≤n≤40) by measuring the time evolution of their sequential monomer or dimer evaporative cooling. The experimental technique, tandem time‐of‐flight mass spectroscopy, measures the relative rate of competing dissociation channels from metastable ion clusters selected during an initial sampling time interval immediately following the creation of the ion cluster ensemble. Pulsed laser UV photoionization converts the distribution of neutral clusters emerging from a free‐jet expansion to the distribution of ion clusters from which the initial selection takes place. For the smaller clusters, 3≤n≤14, we compare the measured dissociation rates with those calculated from a modified version of the RRK theory of unimolecular dissociation. In applying the theory we use monomer and dimer binding energies determined from theoretical calculation. For larger clusters, 15≤n≤40, the binding energies are not known, and we invert ...

Dissociation pathways and binding energies of lithium clusters from evaporation experiments

The unimolecular dissociation of energy rich lithium cluster ions shows that Li+n dissociate by sequential atom or dimer loss. The binding energies of Li+n (n=4–42) generated in an evaporative ensemble are determined from unimolecular decay, within a well defined time window, and energy constraint. They present a sawtooth behavior vs cluster size less pronounced that it should be from a simple metal model. Odd–even alternation is superimposed on the sawtooth behavior, with odd sized cluster ions being more stable. Cohesive energies per atom of Li+n are deduced from these dissociation energies up to n=40 and from extended photo‐induced measurements up to n=95. Cohesive energies per atom of neutral clusters Lin are derived by combining these ionic cohesive energies with the literature ionization potentials. The linearity of the neutral cluster cohesive energy vs the cluster surface to volume ratio permits a volume and a surface energy to be deduced. These values are compared to the bulk values.

Optical excitation in small ionized sodium clusters : closed-shell and open-shell systems

Abstract Photoabsorption cross-section profiles have been obtained for closed-shell and open-shell ionized size-selected sodium clusters Na + 9 , Na + 21 and Na + 11 . The spill-out of the valence electron cloud explains the red-shifted collective excitation frequency with regard to the Mie frequency. Quantum-mechanical calculations may give energy positions of the resonant structures consistent with the experiment. However, the absolute parameters of the profiles suggest that some oscillator strength is missing.

Coulombic fission and evaporation of antimony cluster ions

The preferential dissociation channels of singly and doubly charged antimony clusters have been determined from the unimolecular dissociation of energy rich cluster ions, using an ion stopping technique. It is found that singly charged Sb+n clusters with 5≤n≤80 dissociate by loss of neutral molecules. Neutral dimer loss is observed for n=5, 6, 7 whereas for n≥8, Sb+n loses Sb4. The fragmentation of doubly charged Sb++n has been investigated above the critical size n++c=24 from which doubly charged clusters are detectable in mass spectra. On the time scale of the experiment, which is 1 μs≤t≤100 μs with respect to photoionization, the delayed Coulombic fission into two singly charged clusters competes with the evaporation of Sb4. It is shown that for the smaller Sb++n clusters with 26≤n≤36 the fission to two singly charged clusters is of relatively asymmetrical character, leading to the detachment of five and seven atom cationic fragments. In larger clusters n≥40 the fission is of more symmetrical character...

Thermodynamical properties of ionized lithium oxide clusters, Li2n+pO+n

Lithium oxide clusters Li2n+pOn+ are generated by combining reactive nucleation in a gas aggregation source and photoionization. Unimolecular dissociation of mass selected cluster ions provides evidence that the excess of metal atoms evaporates first leading to the most stable species Li+(Li2O)n, which then evaporate Li2O molecules. The evaporation rate behavior as a function of cluster size demonstrates that Li+(Li2O)n can be prepared with different temperatures. It is discussed how metal evaporation from metal‐rich oxide clusters leads to oxygen saturated clusters with a lower temperature. An estimate of the dissociation energies of Li+(Li2O)n are given for small sizes n≤10 from photoevaporation experiment.

Reactive nucleation of silver clusters with oxygen and water

The nucleation of silver atoms by the gas aggregation technique when adding O2 and H2O molecules in the carrier gas was investigated by photoionization mass spectrometry. No change appeared in the bare silver cluster mass spectra when only water was added to the nucleation carrier gas. In the presence of oxygen and water, reactive nucleation occurred simultaneously. At low reactive gas concentration (unsaturated regime), AgnOxHy compounds with x=2, 3 and 4 are produced for n⩾6. For a larger reactive gas concentration (saturated regime) two series of cluster bunches having Ag3O3Hy as the building block were observed. The precursors of these series are (Ag4O3Hy)+ and (Ag5O5Hy)+, respectively. This behavior differs totally from the results obtained for reactive nucleation of an alkaline vapor but can be viewed as a precursor of the surface silver hydroxidation mechanism.

Photothermodissociation of selenium clusters

The dissociation of selenium cluster ions containing from 7 up to 30 atoms has been studied using unimolecular decay of photothermoexcited clusters. Clusters containing more than 14 atoms evaporate Se6, Se7, and Se8 species, whereas smaller clusters with 7–10 atoms dissociate manly by Se2 loss. When the size increases, Se6 becomes the predominant channel showing a dissociation which evolves toward the bulk behavior. The changes in the observed channels correlate to changes in the dissociation energies which are deduced from the measured fractional dissociation rates and from the entropy contribution. Together with the previous study of tellurium clusters, this work provides comparative behavior of dissociation energies of group VI clusters and their convergence toward bulk properties.

Physical and chemical probes of metallic clusters

Two different approaches for studying the electronic structure of clusters are presented. The physical probe uses the photoexcitation of size selected alkali-atom clusters. Giant resonances govern the absorption cross-section. It is due to the collective excitation of delocalized valence electrons. The behavior of the resonance up to large sizes, 900-atom clusters for potassium and 1500-atom clusters for lithium puts into evidence the failure of the jellium model for lithium. The chemical approach makes use of the nucleation process in the presence of oxygen. From the analysis of mass spectra of oxidized products, informations are gained on the valence of metallic atom in the cluster oxide. The method is applied to europium and thulium clusters. A comparison is done with the simple case of alkali clusters.

Stability of alkali-atom clusters

A detailed study of the stability of alkali-atom clusters is presented and discussed. Free clusters are produced in an unseeded adiabatic expansion and ionized by a pulsed laser beam. A tandem time-of-flight system provides a size selection of ionized species and mass spectrometry of fragments. We have investigated for sodium and potassium clusters containing up to fifty atoms: the metastable decay following ionization, the photo-induced dissociation, the collisional induced dissociation and the collisional charge exchange. Experimental evidence is shown for dissociative process involving evaporation of monomers and dimers. This is explained by the statistical unimolecular theory and by the energetics of the systems.

Photofragmentation of Hot Clusters: Evaporation, Fission, Photofragmentation Spectroscopy

Fragmentation is an ubiquitous phenomenon which occurs in microscopic systems such as nucleons, and in macroscopic objects as well. Despite its destructive nature, the fragmentation gives insight into the intrinsic stability and the binding forces of the fragmented object.