Small Carbon Clusters Research Articles

Polarizability of Small Carbon Cluster Anions from First Principles

We examine the applicability of density functional theory (DFT) to the polarizability of Cn- (n = 3-9) cluster anions. This was achieved by comparing DFT calculations using two different exchange-correlation functionals (the non-empirical local density approximation, LDA, and the semiempirical hybrid functional B97-1) to quantum chemical calculations using the coupled cluster method in the CCSD(T) "gold standard" approximation. We find that, unless the extra electron is not bound at all by DFT, both LDA and B97-1 agree with the CCSD(T) calculation to within 5-10%, allowing for a meaningful qualitative and semiquantitative analysis. Furthermore, the polarizability is found to increase monotonically with chain size, consistent with the trend inferred from electron detachment experiments.

Particle formation from pulsed laser irradiation of soot aggregates studied with a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope

We investigated the physical and chemical changes induced in soot aggregates exposed to laser radiation using a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope to perform near-edge x-ray absorption fine structure spectroscopy. Laser-induced nanoparticle production was observed at fluences above 0.12 J/cm(2) at 532 nm and 0.22 J/cm(2) at 1064 nm. Our results indicate that new particle formation proceeds via (1) vaporization of small carbon clusters by thermal or photolytic mechanisms, followed by homogeneous nucleation, (2) heterogeneous nucleation of vaporized carbon clusters onto material ablated from primary particles, or (3) both processes.

Exotic negative molecules in AMS

“The techniques and equipment developed for AMS studies are well suited for identifying exotic negative ions”. With this sentence begins a pioneering paper by Roy Middleton and Jeff Klein (M&K) on small doubly-charged negative carbon clusters [Nucl. Instr. and Meth. B 123 (1997) 532]. M&K were the first to utilize Accelerator Mass Spectrometry to prove the existence of these clusters and a number of other exotic molecules. We review M&K’s efforts and show how their work is being continued at other laboratories. The latest developments are: (1) the discovery of long-lived molecular hydrogen anions H2-,D2- and (2) the unambiguous identification of the smallest doubly-charged negative molecule (LiF3)2−. In particular we show new experimental data for D3-, and for (LiF3)2−, and we try to answer the question why M&K’s search for this di-anion was unsuccessful.

Energetics and bonding in small lithiated carbon clusters

Small carbon clusters metallized with Li were studied within a hybrid density functional approach with generalized gradient correction. Structures of the ground state and several excited states associated with different isomers and multiplicities were systematically calculated for C x Li y with x = 1–3 and y = 1–5. The most stable isomers are either linear or planar in the ground state. Three-dimensional structures are only identified for CLi 4, CLi 5, and C 3Li 4. There is important charge transfer in these compounds, showing that ionic bonding is favored as the cluster grows in size. Ionization potentials, electron affinities, and the vibrational analysis of all studied states is provided for all clusters. Structural transitions are predicted for C 2Li 2 at 1400 K and C 2Li 4 at 420 K.

Electrostrictive deformations in small carbon clusters, hydrocarbon molecules, and carbon nanotubes

The electrostrictive response of small carbon clusters, hydrocarbon molecules, and carbon nanotubes is investigated using the density functional theory. For ringlike carbon clusters, one can get insight on the deformations induced by an electric field from a simple two-dimensional model in which the positive charge of the carbon ions is smeared out in a circular homogeneous line of charge and the electronic density is calculated for a constant applied electric field within a two-dimensional Thomas-Fermi method. According to the Hellmann-Feynman theorem, this model predicts, for fields of about 1 V/A ring , only a small elongation of the ring clusters in the direction of the electric field. Full three-dimensional density functional calculations with an external electric field show similar small deformations in the ring carbon clusters compared to the simple model. The saturated benzene and phenanthrene hydrocarbon molecules do not experience any deformation, even under the action of relatively intense (1 V/A ring ) electric fields. In contrast, finite carbon nanotubes experience larger elongations ({approx}2.9%) induced by relatively weak (0.1 V/A ring ) applied electric fields. Both C-C bond length elongation and the deformation of the honeycomb structure contribute equally to the nanotube elongation. The effect of the electric fieldmore » in hydrogen terminated nanotubes is reduced with respect to the nanotubes with dangling bonds in the edges.« less

Theoretical study on the electronic absorption spectra of C 3Cl and its ions

Optimized geometries of small carbon clusters C 3Cl and its ions in their ground and selected excited states have been obtained by CASSCF approach. The vertical excitation energies for the electronic excitations to low-lying excited states have been calculated by the CASPT2 method using cc-pVTZ basis set. The predicted vertical excitation energies are in good agreement with available experimental values. The spin–orbit effect on the spin-forbidden transitions is generally small, with oscillator strengths in the magnitude of 10 −7–10 −13. The vertical emission energies for the selected excited states were calculated at CASPT2/cc-pVTZ level of theory based on the CASSCF optimized geometries of the corresponding excited states.

Ionization potentials, dissociation energies and statistical fragmentation of neutral and positively charged small carbon clusters

Dissociation energies, ionization potentials and fragmentation dynamics of neutral, singly- and doubly charged small carbon clusters have been theoretically studied with a combination of the density functional theory, the coupled cluster method and the the statistical model microcanonical Metropolis Monte Carlo. The second ionization potential decreases with the cluster size and is larger than the first one, which also decreases with the size showing oscillations. Dissociation energies also oscillate with the cluster size, being those with an odd number of atoms more stable. C3 cluster has the largest dissociation energy. The combination of a statistical treatment for the cluster fragmentation with experimental results has allowed us to evaluate the energy distribution in collisions experiments.

Study of competing decay processes in small carbon clusters using time-resolved photoelectron imaging

Atomic clusters may often be considered as small pieces of bulk material; however they may also exhibit properties radically different both from the atomic species and from the condensed phase. Photoemission is one of the many powerful methods to investigate clusters and size-effects in such systems, with very often non monotonous size-evolution. In atomic clusters, the finite-size equivalent of photoemission effects can be studied in details: both direct (photoionization or photodetachment, analogous to photoelectric effect) and delayed (analogous to thermionic emission) processes exhibit many features that may differ from the bulk behavior. Time-resolved photoelectron velocity-map imaging is used to analyze these processes in the case of small carbon anion clusters as well as in fullerenes.

Fragmentation of small neutral carbon clusters

We report on theoretical and experimental efforts designed to understand the fragmentation of small neutral carbon clusters. Theoretically, the dissociation dynamics of C n has been investigated using a statistical model based on the microcanonical Metropolis Monte Carlo method. In this model various structural quantities (geometries, dissociation energies, harmonic frequencies…) are required for both the parent cluster and the fragments. They have been obtained from quantum chemistry calculations for C n up to n = 9. Experimentally, a new detection system for high velocity fragments has been recently developed allowing the fragmentation of high velocity clusters to be totally recorded. Results for the branching ratios of deexcitation of C n with 5 ≤ n ≤ 9 formed by electron capture in high velocity C n +–He collisions are presented. In all cases, the agreement between theory and experiment is reasonably good provided that the theoretical branching ratios are convoluted with a C n energy distribution centered at around 10 eV.

Dissociative recombination of small carbon cluster cations

The dissociative recombination of small cluster cations with free, zero-energy electrons was studied at the ASTRID storage ring. It was found that only two-body fragmentation channels contribute: C+C{sub 2} for C{sub 3}{sup +}, C{sub 2}+C{sub 2} and C+C{sub 3} for C{sub 4}{sup +}. For C{sub 5}{sup +} and C{sub 6}{sup +} the final channels were also characterized by two-body fragmentation, but the specific channels were not identified because of limited mass resolution. It is being speculated whether cluster ions larger than C{sub 6}{sup +} may undergo recombination without dissociation. These clusters have a large heat capacity and a correspondingly lower temperature after electron capture. The decay of the intermediate neutral clusters may be influenced by infrared emission, which provides cooling and prevents dissociation.

Electrical characterization of metastable carbon clusters inSiC: A theoretical study

First-principles calculations carried out in $3C$- and $4H\text{\ensuremath{-}}\mathrm{SiC}$ show that small metastable carbon clusters can be created in irradiated $\mathrm{SiC}$. The metastable carbon clusters possess occupation levels in the $p$-type as well as in the $n$-type $4H\text{\ensuremath{-}}\mathrm{SiC}$. Depending on the conditions of irradiation and the annealing temperature new photoluminescence centers might be detected with characteristic local vibrational modes in the spectrum. We provide the occupation levels and the local vibrational modes of the defects to make the identification possible.

Evidence for the Presence of Fullerene in Diesel Soot Using Laser Desorption/Ionization Mass Spectrometry

Intact soot exhausted from diesel engines from a large cargo ship and from a farm tractor, which work with heavy and light oil respectively, has been analyzed using laser desorption/ionization (LDI) mass spectrometry. Caged carbon clusters (or fullerene family) were detected as molecular ions M+· with relatively intense peaks at m/z 600, 720, and 840 corresponding to C50, C60, and C70, respectively. There were also other even number carbon clusters C2n (n=20-142 or higher). Small carbon clusters Cn (n=7-29) with the magic number n=7, 11, 15, 19, 23, and 27 were also observed in the LDI mass spectra. The LDI mass spectra of a fullerene C60 (99%) and intact soot showed definite metastable ion peaks originating from successive C2 loss from molecular ions. It was confirmed that under the same LDI conditions the laser vaporization of graphite never gives any products of the fullerene family or even number carbon clusters.

Approaches to Size-selective Formation of Fullerenes by Cyclization of Highly Reactive Polyyne Chains

Abstract As an approach to size-selective formation of fullerenes, we developed a method based on the cyclization of reactive cyclic polyynes which were generated by [2 + 2] cycloreversion of [4.3.2]propellatriene units. This method was proven to work successfully for the first time for the formation of fullerene ions C60+ and C60− in the gas phase. This protocol is useful for generation of a small three-dimensional carbon cluster ion C36− and a large fullerene C78−, as well.

Ionization cross sections of small cationic carbon clusters in high-energy collisions with helium atoms and stability of multiply charged species

Single, double, triple, and quadruple ionization cross sections of small cationic carbon clusters $\mathrm{C}_{n}{}^{+}$ colliding with helium atoms at a fixed velocity (2.6 atomic units) have been measured. The size ranges from $n=1$ to $n=10$ for single to triple ionization, from $n=5$ to $n=10$ for the quadruple ionization. The dependence of the cross sections with the cluster size is found to be well reproduced by predictions of the independent atom and electron (IAE) collision model. This extends the applicability of this simple model to higher $n$ values and to a higher ionization degree than previously done [M. Chabot et al., Eur. Phys. J. D 14, 5 (2001)]. The branching ratios of multiply charged $\mathrm{C}_{n}{}^{q+}$ clusters remaining intact over a $100\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ time window have been measured ($n=3--10$, $q=2--3$). Branching ratios of nonfragmented doubly charged clusters have been interpreted on the basis of calculated internal energies of $\mathrm{C}_{n}{}^{2+}$ due to single ionization of $\mathrm{C}_{n}{}^{+}$ clusters using the IAE model. This allowed estimates of the minimum energies required to fragment these $\mathrm{C}_{n}{}^{2+}$ species to be derived.

Hypervalent Molecular Cluster: C<sub>28</sub>H<sub>4</sub>

A growing number of recent publications on clusters reflect a tremendous interest in these particles. These studies reveal new fundamental physical and chemical aspects of matter. Clusters are called the fifth state of matter: liquid, solid, cluster, gas and plasma. In this work, a carbon cluster was generated by a spark cluster source and detected by single focusing mass spectrometer in situ. We examined the effects of cluster source parameters on the generation of carbon cluster and report our initial results. This method should be useful for studying the mechanism of fullerene formation. In the case when carbon clusters generated in plasma arc are carried by the Ar or H2 gas flow downstream through a vacuum chamber to the ion source of mass spectrometer, we obtained a small binary carbon cluster C28H4 (hydrogenated fullerene). The empty fullerene is tetravalent and strongly binds four hydrogen atoms, which significantly weakens two different sets of bonds and leads to an open-shell electronic structure. Conclusion is that endohedral C28H4 are hypervalent. We have demonstrated how in situ mass spectrometry has led to the rapid development of an important branch of synthetic fullerene chemistry that has yielded many new small fullerenes and related derivatives with novel structures and properties. The impact of mass spectrometry on the synthesis of fullerene derivatives is the subject of this paper. Significantly, a large fraction of products could be condensed on a specially designed collection plate, which allows further spectroscopic characterization of new derivatives.

COLLISION INDUCED FRAGMENTATION OF FULLERENE CLUSTERS (C60)n

Clusters of C 60 fullerenes are (multi)-ionised in collisions with O 5+ projectiles at a collision energy of 100 keV. The dominant fragmentation channels are analysed by time-of-flight mass spectrometry for different cluster size distributions. Singly charged [Formula: see text], ions are found to be the dominant fragments, in 25% connected with the loss of one or more C 2-units. This result is explained by the large charge mobility in fullerene clusters. Doubly charged fragments, in form of [Formula: see text] and [Formula: see text], contribute to the observed spectrum with less than 5–10% only and are mainly attributed to the fragmentation of dimers. Singly charged small carbon clusters in the size range n = 7 to 19 are formed with low kinetic energies indicating the importance of thermal dissociation processes. The present experiments confirm earlier conclusions on the charge mobility in fullerene clusters.

TIME AND ENERGY-RESOLVED THERMIONIC EMISSION IN CARBON CLUSTERS

We present experimental measurements of time-dependent photoelectron spectra observed in thermionic emission of small carbon cluster anions and C 60 fullerenes. The combination of photoelectron velocity-map imaging with time-gated detection allowed to resolve delayed electron emission at different stages of the emission process. This unique capability allows us to analyze pure thermionic energy spectra under well-defined conditions. Experimental spectra are analyzed within the framework of the detailed balance theory.

Statistical dissociation of small carbon clusters: A phase space theory investigation

Unimolecular fragmentation in small neutral carbon clusters containing 4 or 5 atoms is studied by means of molecular dynamics (MD) simulations and the phase space theory (PST) statistical approach using various rigid body approximations. The clusters are modelled using the Takai–Lee–Halicioglu–Tiller potential, which predicts linear shapes as the most stable structures. The comparison of the distributions of kinetic energy and angular momentum released after dissociation shows that treating the fragments as linear may not be appropriate at high excitation energies. The branching ratios corresponding to the emissions of C2 and C are also calculated.

Are PAHs precursors of small hydrocarbons in photo-dissociation regions? The Horsehead case

We present maps at high spatial and spectral resolution in emission lines of CCH, c-C3H2, C4H, 12CO and C18O of the edge of the Horsehead nebula obtained with the IRAM Plateau de Bure Interferometer (PdBI). The edge of the Horsehead nebula is a one-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal-to-noise ratio in the PDR where intense emission is seen both in the H2 ro-vibrational lines and in the PAH mid-infrared bands. C18O peaks farther away from the cloud edge. Our observations demonstrate that CCH, c-C3H2 and C4H are present in UV-irradiated molecular gas, with abundances nearly as high as in dense, well-shielded molecular cores. PDR models i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H2 peaks; and ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV-irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amounts.

Measurement of carbon cluster charge state passing through thin foils using a luminance plate

A measurement system based on Coulomb explosion imaging for analyzing the structure of small carbon clusters has been developed. A cluster ion accelerated to the MeV energy region passed through thin foil, and the fragment atoms exploded with Coulomb repulsion were detected with a luminance screen detector. The trace image of the fragment atoms on the screen of the detector monitored by the CCD camera was downloaded as digital data to a hard disc. In preliminary experiments, both chain and ring structures were observed for C 3 and C 6, respectively. In addition, we measured the average charge states of constituent atoms of the carbon cluster after it passed through the thin foil; these charges are related to the cluster structure.