Photodetachment spectroscopy of cold aluminum cluster anions

Identification of Magic Numbers in Homonuclear Clusters: The ε3 Stability Ranking Function

With the rise of cluster-assembled materials, an index that is able to rank and identify stable clusters or molecules is of great interest in materials sciences and engineering. In the present work, we applied a stability ranking function (ε³) in nanoclusters formed by simple metals (Na, Mg), main group elements (Al), or transition metals (Ti, Cu). The ε³ function parameters are molecular properties derived from the wave function. These parameters can be divided into kinetic and thermodynamic descriptors, in which the kinetic descriptors are the ionization potential and electronic excitation energy, while the atomization free Gibbs energy is the thermodynamic one. This simple ε³ function was able to identify the possible magic numbers of the studied clusters across the periodic table in a good agreement with previous experimental and theoretical works.

TGMin: An efficient global minimum searching program for free and surface-supported clusters

In this article, we introduce an efficient global-minimum structural search program named Tsinghua Global Minimum 2 (TGMin-2), which is the successor of the original TGMin algorithm that was developed in our group in 2011. We have introduced a number of new features and improvements into TGMin-2, including a symmetric structure generation algorithm that can produce good initial seeds for small- and medium-size clusters, the duplicated structure identification algorithm, and the improved structure adaption algorithm that was implemented in the original TGMin code. To predict the simulated photoelectron spectrum (PE spectrum) automatically, we also implemented a standalone program named AutoPES (Auto Photoelectron Spectroscopy), which can be used to simulate PE spectra and compare them with experimental results automatically. We have demonstrated that TGMin-2 and AutoPES are powerful tools for studying free and surface-supported molecules, clusters, and nanoclusters. © 2018 Wiley Periodicals, Inc.

Computational cogitation of Cn@Al12 clusters

A variety of novel Cn Al12 core-shell nanoclusters have been investigated using density functional calculations. A series of Cn cores (n=1-4) have been encapsulated by icosahedral Al12 , with characteristic physical properties (energetics and stabilities, ionisation energies, electron affinities) calculated for each cluster. Other isomers, with the Cn moiety bound externally to the Al12 shell, have also been studied. For both series, a peak in stability was found for n(C)=2, a characteristic that appears to be inextricably linked with the relaxation of the constituent parts upon dissociation. Analysis of trends for ionisation energies and electron affinities includes evaluation of contributions from the carbon and aluminium components, which highlights the effects of composition and morphology on cluster properties.

Al4- cluster anion: electronic structure, excited states, and electron detachment

Full-valence complete-active-space self-consistent-field and coupled-cluster methods are used to investigate excitation and electron detachment energies of the Al(4)(-) cluster anion. Both, Al(4) and Al(4)(-), have electronic structures with many low-energy electron configurations and exhibit accordingly pronounced configuration mixing. The complete-active-space self-consistent-field calculations show that all low-energy states of the neutral and the cluster anion can nevertheless be characterized by a single configuration, and in contrast to Møller-Plesset perturbation theory, coupled-cluster methods are found to recover enough static correlation to describe these states accurately. Coupled-cluster methods are then used to find minimal-energy structures of four states of the anion and four states of the neutral cluster, the respective ground states are identified, and vertical and adiabatic detachment energies are computed. The computed vertical detachment energies suggest that the first peak in the experimental photoelectron spectrum consists of at least three different transitions, and the computed excitation energies corroborate the conclusions from the analysis of the recently observed delayed detachment from excited Al(4)(-) ions in an electrostatic trap.

Novel gas-stabilized iron clusters: synthesis, structure and magnetic behaviour

A novel method for the synthesis of nanostructured films produced by depositing gas-phase magnetic nanoparticles is presented and the properties of the films are reported. The technique mixes metal vapour and small argon clusters produced in a supersonic expansion. The condensed clusters are subsequently deposited in situ onto copper grids. The cluster size is controlled by the vapour pressure of the metal inside the pick-up chamber. Detailed analysis of the transmission electron micrographs of the Fe clusters shows that there is a simple linear relationship between the average metal cluster diameter and the metal vapour pressure during deposition. Furthermore, the nanoparticles show a relatively narrow size distribution for a given set of experimental conditions. Structural and magnetic investigations have been performed on Fe cluster samples, and the influence of the metal vapour pressure has been studied. Detailed analysis of the magnetic and structural data has been performed and valuable information such as cluster size distributions, strength of the interparticle dipolar interactions and average magnetic moment per cluster are derived. It is shown that, at room temperature, the magnetic behaviour of the films is consistent with nanoparticle supermoments interacting via dipolar interactions.

Photoelectron spectroscopy of AlnD2− (n=3–15): Observation of chemisorption and physisorption of dideuterium on aluminum cluster anions

Photoelectron spectroscopy is used to investigate aluminum dideuteride cluster anions, AlnD2- (n=3,6-15), produced by laser vaporization of a pure Al target with a D2-seeded helium carrier gas. Comparison between the well-resolved photoelectron spectra of AlnD2- and Aln- reveals the nature of interactions between D2 and Aln-. Depending on the size of the Aln- clusters and their electronic structure, three types of AlnD2- species are observed, dideuteride (dissociative chemisorption), molecular chemisorption, and physisorption. Striking spectral similarities are observed between photoelectron spectra of AlnD2- and Aln- for n=9, 11, 13, and 15, suggesting that D2 is physisorbed on these closed-shell Aln- clusters. For AlnD2- with n=3, 6, 7, and 10, completely different spectra are observed in comparison with the corresponding Aln- clusters, suggesting that the AlnD2- species may be characterized as dideuterides. For AlnD2- with n=8, 12, and 14, in which the Aln- clusters are open shell, the D2 is characterized as chemisorption on the basis of spectral shifts and similarities relative to those of the corresponding Aln- clusters.

METAL TO INSULATOR TRANSITIONS IN CLUSTERS

The strict criterion for metallicity, a finite density of states (DOS) at the Fermi energy (E(F)), cannot be applied to clusters because energy levels are always discrete in a system of finite size. We propose an alternative definition whereby clusters can be considered metallic when the gap between occupied and unoccupied states at E(F) is consistently smaller than or equal to the Kubo band gap delta. We use the experimental findings of photoelectron spectroscopy of anionic clusters to analyze band gaps of various cluster families. Monovalent clusters (alkali and noble metals) grossly follow the shell structure pattern, producing band gaps smaller than delta for most cluster sizes, with some exceptional sizes exhibiting electronic shell closure or symmetry-induced band gaps. Among the bivalent metals, only mercury shows consistent band gap closure with increasing cluster size, that is a simple insulator-metal transition. Other bivalent elements such as Zn and Mg exhibit a much more complicated behavior. We also briefly discuss complex cluster families such as aluminum and transition metals.

Study of the stabilization energies of halide-water clusters: An application of first-principles interaction potentials based on a polarizable and flexible model

The aim of this work is to compute the stabilization energy E(stab)(n) of [X(H(2)O)(n)](-) (X identical with F, Br, and I for n=1-60) clusters from Monte Carlo simulations using first-principles ab initio potentials. Stabilization energy of [X(H(2)O)(n)](-) clusters is defined as the difference between the vertical photodeachment energy of the cluster and the electron affinity of the isolated halide. On one hand, a study about the relation between cluster structure and the E(stab)(n) value, as well as the dependence of the latter with temperature is performed, on the other hand, a test on the reliability of our recently developed first-principles halide ion-water interaction potentials is carried out. Two different approximations were applied: (1) the Koopmans' theorem and (2) calculation of the difference between the interaction energy of [X(H(2)O)(n)](-) and [X(H(2)O)(n)] clusters using the same ab initio interaction potentials. The developed methodology allows for using the same interaction potentials in the case of the ionic and neutral clusters with the proviso that the charge of the halide anion was switched off in the latter. That is, no specific parametrization of the interaction potentials to fit the magnitude under study was done. The good agreement between our predicted E(stab)(n) and experimental data allows us to validate the first-principles interaction potentials developed elsewhere and used in this study, and supports the fact that this magnitude is mainly determined by electrostatic factors, which can be described by our interaction potentials. No relation between the value of E(stab)(n) and the structure of clusters has been found. The diversity of E(stab)(n) values found for different clusters with similar interaction energy indicates the need for statistical information to properly estimate the stabilization energy of the halide anions. The effect of temperature in the prediction of the E(stab)(n) is not significant as long as it was high enough to avoid cluster trapping into local equilibrium configurations which guarantees an appropriate sampling of the configurational space. Parallel tempering method was applied in particular cases to guarantee satisfactory sampling of clusters at low temperature.

Isomer-resolved ion spectroscopy

We demonstrate the isomer-resolved spectroscopy of gas-phase ions, with different geometries separated prior to spectroscopic probe using ion mobility techniques. Specifically, ring and chain isomers of carbon cluster anions with 10;-12 atoms have been separated by ion mobility/mass spectrometry and examined by photoelectron spectroscopy. This methodology should also apply to other ion spectroscopies, including IR photodissociation.

Direct observation of the dynamics of electronic excitations in molecules and small clusters

Femtosecond time-resolved photoelectron spectroscopy is applied to study relaxation paths of excited states of mass-selected negatively charged clusters. As a first example, the lifetime of an excited state of the carbon trimer anion is measured directly. In addition, the mechanism of the decay, i.e., the configurations of the participating electronic states, is determined from the photoelectron spectra. In general, this method can be used to study all kinds of electronic excitation and relaxation processes in mass-selected nanoparticles.