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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...

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.

Rare earth clusters of europium, thullium and ytterbium were generated by gas aggregation technique and probed by photoionization mass spectrometry. Their relative intensities in mass spectra have shown that their stabilities are governed by compact geometrical structures. The addition of oxygen gas in the nucleation region was used to produce the reactive nucleation. Several stages of oxidation were observed as a function of oxygen pressure up to saturation. For the maximal degree of oxidation the observed oxide ion compositions enable one to follow the valence of metal atom in its oxide as cluster size increases. This exhibits a divalent to trivalent valence change with cluster size. Moreover it emerges from the data that the divalent to trivalent transition for Tm, Yb, and Eu occurs at different size values.

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.

Alkali clusters arc considered as prototypes of metallic clusters since their electrons arc delocalized from the small sizes. The two techniques applied here I-c- photoionization and its reverse neutralization by charge exchange probe the delocalization of their valence electrons. These methods extending over other elements should produce a pertinent analyse of the binding evolution with cluster size.

Metal cluster provides an ideal object to study a many body system. In particular it consists of many electrons which strongly interact each other. Two kinds of electrons may be distinguished i.e.: the valence electrons which become more delocalized as the cluster size increases, and the core electrons which may remain localized around each nucleus whatever the cluster size is. The correlations between the electrons, within each variety, induce collective effects revealed by the observation of giant resonances.

Shell and supershell structures have been observed for lithium clusters produced in a gas-aggregation type source, up to the size range n≃2000. Noteworthy similarities are found as compared with observations performed on Nan clusters. Thermal effects are briefly investigated.

The photoionization of antimony clusters has been studied in the energy range 20-120 eV, probing the inner-valence 4d-np transitions as well as the shape resonance 4d-∈f. When the cluster size is varied the photoionization efficiency presents a quite surprising behavior. For all clusters, except the Sb 4p group with p>1, the photoionization pattern is similar to that which is known for the bulk. On the other hand, the shape resonance collapses completely for Sb 4p

Photofragmentation studies are described for size‐selected ionized potassium‐atom clusters, generated in a supersonic expansion and analyzed with a tandem time‐of‐flight system. In the range n=3 to 41 the relative ion intensities in photofragmentation spectra reflect the relative stabilities of the ion products. Moreover the photon‐energy dependence of the fragmentation patterns suggests that photodissociation proceeds rather via a fast sequential evaporation of neutral monomers than via a fission process. A value can be deduced for the mean binding energy per atom which is increasing with the cluster size.

Neutral potassium clusters of two different mass distributions generated either by a sonic or a conical nozzle were photoionized at several ionizing energies. By combining different nucleation conditions with several photoionization situations, variations of ion mass spectral patterns reflecting either neutral or ionic stabilities were obtained. The spectra show peaks or steps for those neutral and ionized clusters which contain 8, 20, and 40 electrons. These can be understood in terms of electronic shell structure for both neutral and ionized potassium clusters. However local irregularities appearing for the small cluster sizes reveal the influence of their geometrical structure. Moreover ionizing laser power dependencies taken at various photon energies were used to study fragmentation effects following ionization.

The unimolecular dissociation of metastable photoionized mass‐selected heterogeneous alkali clusters is investigated using a tandem time‐of‐flight spectrometer. NaK+n and Nan K+ are found to dissociate by evaporation of either a single neutral atom or a neutral dimer of the most abundant constituant. This behavior differs from the evaporation of heterogeneous neutral clusters which might always evaporate potassium in order to explain the sodium enrichment in the NaxKy neutral cluster distributions. This difference in the dissociation patterns of neutral and ionic heterogeneous clusters puts into evidence the influence of the charge in unimolecular dissociation processes.

The decomposition of metastable photoionized mass‐selected alkali clusters is investigated using a tandem time‐of‐flight spectrometer. Na+n and K+n are found to decompose mainly by the evaporation of either a single neutral atom or a neutral dimer in a time scale of about 10 μs. The predominant fragmentation channels are found to follow the adiabatic dissociation channels associated with the lowest energies accordingly to our CI calculations. In the light of our experimental and theoretical results a comparison between the different calculations of the absolute atomization energies available in the literature is presented.

Photoionization of small K n and NaK n −1 clusters with n ⩽ 34 has been achieved in a crossed laser-molecular-beam experiment. The relative ion peak intensities in mass spectra reveal fragmentation effects which are wavelength-dependent. The vertical ionization profiles in the threshold region give some insight into the electronic structure.

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.