B. Baguenard

Evidence for icosahedral atomic shell structure in nickel and cobalt clusters. Comparison with iron clusters

Abstract For the first time, the mass distribution of nickel, cobalt and iron clusters is analyzed in a large mass range with near threshold photoionization experiments and standard time-of-flight mass spectrometry. In the case of nickel and cobalt, oscillations observed in mass spectra correspond to icosahedral atomic shell structure in the studied mass range (50–800 atoms). For iron clusters, the situation is less clear. The exact location of structures in mass spectra depends on source operating conditions. We have observed the competition between different cluster geometries.

Shell structure in photoionization spectra of large aluminum clusters

Photoionization mass spectrometry experiments, performed on an extensive size range of aluminum clusters produced by laser vaporization technique, are reported. Ionization potential values are deduced from individual photoionization efficiency curves for the smaller AlN clusters (N=36–112). Our results confirm and complete those previously published. The mass spectra of larger clusters (N≊250–1400) reveal a regular signal oscillation. Several additional experiments give proof that this striking pattern originates from size‐dependent ionization threshold effects. This structure exhibits exact periodicity as a function of N1/3 or Ne1/3 (Ne the number of valence electrons).

Shell structure of small indium clusters below N≈200 atoms

Abstract Neutral indium cluster In N ( N ≤200) are laser photoionized near threshold and analyzed by time-of-flight mass spectrometry. Strong shell effects are observed in cluster mass spectra. For In N clusters below N =125, individual ionization potentials are deduced. They are compared with theoretical predictions calculated within the framework of the spherical-jellium model. The agreement is qualitatively quite good and the observed shell effects correspond to the successive major electronic shell closures. Deviations from electronic shell model predictions are much less important for indium clusters than for aluminum ones. A tentative explanation of this difference is given.

Competition between atomic shell and electronic shell structures in aluminum clusters

Under usual experimental conditions, aluminum clusters have specific geometric arrangement. By heating the nozzle, we obtain melted aluminum clusters, and a new periodicity appears in mass spectra, which corresponds to electronic shells up to 1800 electrons.

Unimolecular dissociation of trivalent metal cluster ions: The size evolution of metallic bonding

The unimolecular decomposition of size selected cluster cations of trivalent metals (Aln+, Gan+, and Inn+), induced by high fluence laser ionization, has been investigated in the n=7 to n=85, 55, and 75 size ranges, respectively. This method is applied for the first time to photoexcited trivalent clusters generated in an evaporative ensemble and the experimental data cover a size range that was not explored in previous pioneering experiments on their dynamics. Small clusters dissociate through the loss of a neutral or a charged atom whereas clusters larger than a well defined critical size merely dissociate through the first channel. In the framework of the RRK statistical theory, the measured evaporation rates provide some information about the size evolution of the cluster dissociation energies and their ionization potentials in the low size range. The competition between the ion and the atom evaporation is found to be consistent with the size evolution of the ionization potentials independently measure...

Decay of C60 by delayed ionization and C2 emission: Experiment and statistical modeling of kinetic energy release

C(60) molecules highly excited in the nanosecond regime decay following ionization and dissociation by emitting a series of carbon dimers, as well as other small fragments if excitation is strong enough. The fragmentation mass spectrum and kinetic energy release of all charged fragments obtained in these experiments are interpreted within the framework of the Weisskopf theory, using a realistic Monte Carlo procedure in which the rates of all relevant decay channels are modeled using Arrhenius expressions. Comparison between the measurements and the simulated spectra allows the distribution of deposited energy to be accurately estimated. The dependence of the fragment kinetic energies on the laser fluence, found in the simulation but not observed in the experimental results, indicates that the small fragments are not necessarily emitted from small fullerenes resulting from C(60) by sequential decay. Rather, direct multifragmentation of C(60) is invoked to interpret the observed patterns. The possible role of post-ionization of neutral emitted fragments is discussed.

Shell effects in photoionization spectra of heavy trivalent metal clusters

Laser photoionization experiments have been performed on a large size range of indium and thallium clusters. The metal clusters were produced by laser vaporization technique and analyzed after laser ionization by standard time-of-flight mass spectrometry. For the indium clusters, individual ionization potentials (IP) are deduced for N≤132. Abrupt decreases in the IP values are observed, which correspond to the openings of new electronic shell as predicted by the spherical jellium model. For larger indium clusters, the unresolved mass spectra present small but reproducible oscillations. Interpretations in terms of either electronic shell structure or cluster geometry remain undecided for the moment. Our results on thallium clusters are less abundant and only qualitative. Nevertheless, they show that thallium behaves more like a monovalent element than like a trivalent one.

Electronic Shells and Supershells in Gallium and Aluminum Clusters

As first shown by Knight et al on sodium clusters NaN[1] the size-dependence of simple-metal cluster properties are to a large extent correlated to the electronic shell structure of their conduction electrons. In the early stage alkali species have been extensively studied because of their simple atomic and bulk electronic structures and the ability to produce intense cluster beams of these low melting-point elements [2,3]. Subsequently the shell structure has been observed in mass spectra of more complex elements, where ion-pseudopotential and band-structure effects are known to be stronger in the bulk, such as noble and group-II B elements, and trivalent group-IIII A metals (s2p1 atomic configuration) [2,3]. A comparative analysis of various mass spectra reported in the literature clearly proves that the development of the electronic shell structure in large clusters requires experimental conditions ensuring the melting -or at least the surface melting- of the ionic frame [4]. As long as only the qualitative features of the electronic shell structure are concerned, the theoretical description of the electronic cloud interacting with the ionic background may be merely reduced to the problem of Ne=vN independent itinerant electrons trapped in a spherical flatbottom potential well (v is the valence number). The shell structure is simply the direct consequence of the non-smooth quantized bound spectrum in the finitesize confining potential. Most of the theoretical and experimental works aim to determine the characteristic sizes related to the shell structure and to locate the size range where the geometric effects or the bulk behaviour begin to prevail. As it will be shown below this size range strongly depends on the experimental conditions.

Shell effects in large AlN clusters

Neutral aluminum clusters have been produced by laser vaporization technique, ionized by a low-power near-threshold laser light and detected using standard TOF spectrometric methods. Ionization potentials have been deduced in the low size range. In the large size range 250<N<1400 the patterns of the mass spectra exhibit a regular and continuous oscillation, originating from size-dependent ionization threshold effects. The period, constant on a Ne1/3 scale (Ne=3N is the number of valence electrons), is approximately two times shorter than the one observed in alkali experiments. This feature is analyzed in terms of shell structure.

Atomic shell structures observed in photoionization spectra of nickel and cobalt clusters

Nickel and cobalt clusters have been studied by near threshold laser-photoionization and time-of-flight mass spectrometry. In the size domain from 50 up to 800 atoms, the mass distributions of the photoionized products look very similar for nickel and cobalt clusters. In both cases a regular structure is observed which is periodic on a N1/3 scale. It is found to be consistent with the filling of successive icosahedral shells of atoms. The recurring details of this structure agree with the so-called umbrellas model.