When Mass Spectrometry Meets Nanoparticles. Weighing, Manipulating and Fragmenting Single Nanoparticles in The Gas Phase

A new mass spectrometer based on charge detection has been developed. It enables the determination of mass distribution of various macromolecules and nanoparticles. By using an ion trap coupled to a CO2 laser, photoinduced dissociation of such macroions can be studied at the single ion level giving uniquely access to intrinsic features such as fragmentation patterns and unimolecular dissociation activation energy

When Mass Spectrometry Meets Nanoparticles. Weighing, Manipulating and Fragmenting Single Nanoparticles in The Gas Phase

A new mass spectrometer based on charge detection has been developed. It enables the determination of mass distribution of various macromolecules and nanoparticles. By using an ion trap coupled to a CO2 laser, photoinduced dissociation of such macroions can be studied at the single ion level giving uniquely access to intrinsic features such as fragmentation patterns and unimolecular dissociation activation energy

Statistical evaporation of rotating clusters. IV. Alignment effects in the dissociation of nonspherical clusters

Unimolecular evaporation in rotating, non-spherical atomic clusters is investigated using Phase Space Theory in its orbiting transition state version. The distributions of the total kinetic energy release epsilon_tr and the rotational angular momentum J_r are calculated for oblate top and prolate top main products with an arbitrary degree of deformation. The orientation of the angular momentum of the product cluster with respect to the cluster symmetry axis has also been obtained. This statistical approach is tested in the case of the small 8-atom Lennard-Jones cluster, for which comparison with extensive molecular dynamics simulations is presented. The role of the cluster shape has been systematically studied for larger, model clusters in the harmonic approximation for the vibrational densities of states. We find that the type of deformation (prolate vs. oblate) plays little role on the distributions and averages of epsilon_tr and J_r except at low initial angular momentum. However, alignment effects between the product angular momentum and the symmetry axis are found to be significant, and maximum at some degree of oblateness. The effects of deformation on the rotational cooling and heating effects are also illustrated.Comment: 15 pages, 9 figure

Thermal expansion in small metal clusters and its impact on the electric polarizability

The thermal expansion coefficients of $\mathrm{Na}_{N}$ clusters with $8 \le N \le 40$ and $\mathrm{Al}_{7}$, $\mathrm{Al}_{13}^-$ and $\mathrm{Al}_{14}^-$ are obtained from {\it ab initio} Born-Oppenheimer LDA molecular dynamics. Thermal expansion of small metal clusters is considerably larger than that in the bulk and size-dependent. We demonstrate that the average static electric dipole polarizability of Na clusters depends linearly on the mean interatomic distance and only to a minor extent on the detailed ionic configuration when the overall shape of the electron density is enforced by electronic shell effects. The polarizability is thus a sensitive indicator for thermal expansion. We show that taking this effect into account brings theoretical and experimental polarizabilities into quantitative agreement.Comment: 4 pages, 2 figures, one table. Accepted for publication in Physical Review Letters. References 10 and 23 update

Emergence of Bulk CsCl Structure in (CsCl)nCs+ Cluster Ions

The emergence of CsCl bulk structure in (CsCl)nCs+ cluster ions is investigated using a mixed quantum-mechanical/semiempirical theoretical approach. We find that rhombic dodecahedral fragments (with bulk CsCl symmetry) are more stable than rock-salt fragments after the completion of the fifth rhombic dodecahedral atomic shell. From this size (n=184) on, a new set of magic numbers should appear in the experimental mass spectra. We also propose another experimental test for this transition, which explicitely involves the electronic structure of the cluster. Finally, we perform more detailed calculations in the size range n=31--33, where recent experimental investigations have found indications of the presence of rhombic dodecahedral (CsCl)32Cs+ isomers in the cluster beams.Comment: LaTeX file. 6 pages and 4 pictures. Accepted for publication in Phys. Rev.

Structural Transitions and Global Minima of Sodium Chloride Clusters

In recent experiments on sodium chloride clusters structural transitions between nanocrystals with different cuboidal shapes were detected. Here we determine reaction pathways between the low energy isomers of one of these clusters, (NaCl)35Cl-. The key process in these structural transitions is a highly cooperative rearrangement in which two parts of the nanocrystal slip past one another on a {110} plane in a direction. In this way the nanocrystals can plastically deform, in contrast to the brittle behaviour of bulk sodium chloride crystals at the same temperatures; the nanocrystals have mechanical properties which are a unique feature of their finite size. We also report and compare the global potential energy minima for (NaCl)NCl- using two empirical potentials, and comment on the effect of polarization.Comment: extended version, 13 pages, 8 figures, revte

Optical extinction and scattering cross sections of plasmonic nanoparticle dimers in aqueous suspension

Absolute extinction and scattering cross sections for gold nanoparticle dimers were determined experimentally using a chemometric approach involving singular-value decomposition of the extinction and scattering spectra of slowly aggregating gold nanospheres in aqueous suspension. Quantitative spectroscopic data on plasmonic nanoparticle assemblies in liquid suspension are rare, in particular for particles larger than 40 nm, and in this work we demonstrate how such data can be obtained directly from the aggregating suspension. Our method can analyse, non invasively, the evolution of several sub-populations of nanoparticle assemblies. It may be applied to other self-assembling nanoparticle systems with an evolving optical response. The colloidal systems studied here are based on 20, 50 and 80 nm gold nanospheres in aqueous solutions containing sodium lipoate. In these systems, the reversible dimerisation process can be controlled using pH and ionic strength, and this control is rationalised in terms of DLVO theory. The dimers were identified in suspension by their translational and rotational diffusion through scattering correlation spectroscopy. Moreover, their gigadalton molecular weight was measured using electrospray charge-detection mass spectrometry, demonstrating that mass spectrometry can be used to study nanoparticles assemblies of very high molecular mass. The extinction and scattering cross sections calculated in the discrete-dipole approximation (DDA) agree very well with those obtained experimentally using our approach

Ab initio many-body calculations on infinite carbon and boron-nitrogen chains

In this paper we report first-principles calculations on the ground-state electronic structure of two infinite one-dimensional systems: (a) a chain of carbon atoms and (b) a chain of alternating boron and nitrogen atoms. Meanfield results were obtained using the restricted Hartree-Fock approach, while the many-body effects were taken into account by second-order M{\o}ller-Plesset perturbation theory and the coupled-cluster approach. The calculations were performed using 6-31$G^{**}$ basis sets, including the d-type polarization functions. Both at the Hartree-Fock (HF) and the correlated levels we find that the infinite carbon chain exhibits bond alternation with alternating single and triple bonds, while the boron-nitrogen chain exhibits equidistant bonds. In addition, we also performed density-functional-theory-based local density approximation (LDA) calculations on the infinite carbon chain using the same basis set. Our LDA results, in contradiction to our HF and correlated results, predict a very small bond alternation. Based upon our LDA results for the carbon chain, which are in agreement with an earlier LDA calculation calculation [ E.J. Bylaska, J.H. Weare, and R. Kawai, Phys. Rev. B 58, R7488 (1998).], we conclude that the LDA significantly underestimates Peierls distortion. This emphasizes that the inclusion of many-particle effects is very important for the correct description of Peierls distortion in one-dimensional systems.Comment: 3 figures (included). To appear in Phys. Rev.

Work functions, ionization potentials, and in-between: Scaling relations based on the image charge model

We revisit a model in which the ionization energy of a metal particle is associated with the work done by the image charge force in moving the electron from infinity to a small cut-off distance just outside the surface. We show that this model can be compactly, and productively, employed to study the size dependence of electron removal energies over the range encompassing bulk surfaces, finite clusters, and individual atoms. It accounts in a straightforward manner for the empirically known correlation between the atomic ionization potential (IP) and the metal work function (WF), IP/WF$\sim$2. We formulate simple expressions for the model parameters, requiring only a single property (the atomic polarizability or the nearest neighbor distance) as input. Without any additional adjustable parameters, the model yields both the IP and the WF within $\sim$10% for all metallic elements, as well as matches the size evolution of the ionization potentials of finite metal clusters for a large fraction of the experimental data. The parametrization takes advantage of a remarkably constant numerical correlation between the nearest-neighbor distance in a crystal, the cube root of the atomic polarizability, and the image force cutoff length. The paper also includes an analytical derivation of the relation of the outer radius of a cluster of close-packed spheres to its geometric structure.Comment: Original submission: 8 pages with 7 figures incorporated in the text. Revised submission (added one more paragraph about alloy work functions): 18 double spaced pages + 8 separate figures. Accepted for publication in PR

Surface reconstruction induced geometries of Si clusters

We discuss a generalization of the surface reconstruction arguments for the structure of intermediate size Si clusters, which leads to model geometries for the sizes 33, 39 (two isomers), 45 (two isomers), 49 (two isomers), 57 and 61 (two isomers). The common feature in all these models is a structure that closely resembles the most stable reconstruction of Si surfaces, surrounding a core of bulk-like tetrahedrally bonded atoms. We investigate the energetics and the electronic structure of these models through first-principles density functional theory calculations. These models may be useful in understanding experimental results on the reactivity of Si clusters and their shape as inferred from mobility measurements.Comment: 9 figures (available from the author upon request) Submitted to Phys. Rev.