Carbon Cluster Anions Research Articles

Electron detachment spectroscopic study on carbon and silicon cluster anions

The electron detachment of mass-selected carbon and silicon cluster anions was studied using different ion-source conditions. Ultraviolet photoelectron spectra of medium-sized carbon cluster anions were measured using a laser vaporization source with different fluences. On systematic inspection, the series of spectra showed 4n-periodicity in their electron affinity, which is consistent with the predicted ring-form structure. For silicon cluster anions, resonance-enhanced multiphoton electron detachment spectra were measured. The role of the carrier gases He, N 2 and CO 2 is discussed.

Time and kinetic energy resolved delayed electron emission in small carbon cluster anions

We present the first study of time-resolved delayed electron emission in small anionic carbon clusters. Using photoelectron velocity map imaging spectroscopy in conjunction with time-gated detection we are able to resolve delayed electron emission at different stages of the emission process. This unique capability is an essential tool to analyse pure thermionic energy spectra under well-defined conditions. The more striking result of the present work is that the effective temperature at a given delay after excitation is determined by the emission rate, rather than by the excitation energy deposited in the cluster.

Velocity-map imaging electron spectrometer with time resolution

A velocity-map imaging system for time-resolved photoelectron spectroscopy is presented and discussed. Nanosecond gating of microchannel plates synchronized with the excitation laser in a velocity-map imaging spectrometer has been used to generate time-resolved images of electrons emitted from photoexcited xenon atoms and small carbon cluster anions. These images represent a time slice in the kinetic energy spectrum and reveal the differing time scales of competing electron emission processes.

H-substituted anionic carbon clusters CnH− (n⩽10): Density functional studies and experimental observations

We have studied the interaction of hydrogen with small neutral and anionic carbon clusters using density functional calculations. The geometry, stability, and electronic structure of these clusters show an odd–even alternation originating in the bonding nature of the carbon atoms. Our mass spectrometric measurements of the abundance of CnH− (n⩽10) cluster anions produced by gas-feed Cs sputtering from different crystallographic forms of carbon display similar odd–even alternation with the even-n clusters being relatively more abundant. The calculated trend in the adiabatic electron affinities shows a behavior similar to the experimental abundance pattern. We discuss a possible partial suppression of the chain-to-ring transformation (which normally occurs at n=10 in Cn−) in CnH− and compare it with our density functional calculations as well as observations in CnN−. We also observe that the size dependence of the abundance of CnH− clusters sputter ejected from a fullerene target exhibits a distinctly different power-law decline compared to crystalline and amorphous carbon.

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.

Small carbon clusters (C n0, C n+, C n−) from acyclic and cyclic precursors : Neutralization–reionization and theory

Dissociative ionization of several linear, cyclic, and branched molecules was used to generate C 3 + and C 4 + carbon clusters. However, mass spectra from collisionally activated dissociation (CAD) and neutralization–reionization (NR) under a wide variety conditions were indistinguishable, indicating only one isomer or same mixture of isomers. Likewise, CAD and NR spectra of C 4 + and C 3 + from 13 CH 2 CHCH 13 CH 2 and CH 3 13 C CH 2 show complete 12 C / 13 C scrambling. CAD cross sections are consistent with C 4 +–C 6 + ions as mainly linear isomers and C 7 + ions from cyclic precursors as mainly cyclic. CCSD(T) ab initio and B3LYP density functional theory calculations with large basis sets yielded structures and energies for a variety of cationic, neutral, and anionic carbon clusters and transition states for some cationic rearrangements. The calculated enthalpies of formation, dissociation energies, ionization energies, and electron affinities mostly agree with currently accepted experimental data. However, CCSD(T) calculations indicate adiabatic ionization energies for C 3 (11.79 eV) and C 4 isomers (10.48, 10.82, and 10.86 eV) that differ >1 eV from recent measurements.

Thermionic emission in small carbon cluster anions

Photodetachment of mass-selected small C n − clusters has been studied by photoelectron imaging spectroscopy. As in small refractory metal cluster anions, the low kinetic energy spectrum is dominated by thermionic emission. However, the effective temperatures measured from the electron kinetic energy spectra are sensibly larger than expected assuming complete thermal equilibrium. This discrepancy, more significant than in metal clusters, is attributed to the presence of a large gap in the density-of-states, which precludes a complete thermal equilibrium in the excited species before decay.

Electronic relaxation dynamics of carbon cluster anions: Excitation of the C̃ 2Πg←X̃ 2Πu transition in C6−

Anion femtosecond photoelectron spectroscopy (FPES) has been used to monitor intramolecular electronic relaxation dynamics following the excitation of the C̃ 2Πg←X̃ 2Πu 000 electronic transition in C6−. The time-dependent photoelectron spectra provide a detailed picture of the relaxation dynamics in which the initially excited C̃ 2Πg (v=0) level evolves into highly vibrationally excited C6− in its ground electronic state. The spectra show evidence for a two-step relaxation mechanism: internal conversion (IC) to vibrationally excited B̃ 2Σu+ and à 2Σg+ states, occurring on a time scale of 730±50 fs, followed by IC from these intermediate states to highly vibrationally excited levels in the X̃ 2Πu ground state with a time constant of 3.0±0.1 ps.

Formation of nitrogen-substituted carbon cluster anions by gas-feed Cs-sputtering from different forms of carbon

We have investigated the mechanism of formation of Cn - and CnN- cluster anions during Cs+-ion sputtering from surfaces of different crystallographic forms of carbon (polycrystalline graphite, amorphous C and C60), in the presence of high purity N2 or NO2 gas. The normalised yields of sputter-ejected CnN- (n? 10) cluster anions display an oscillatory dependence on n: the odd n clusters being more abundant than the even n ones. Significantly, CnN- clusters produced from the three different forms of C, show similar oscillatory variations with n. Moreover, the CnN- yields follow a single power law decline as a function of cluster size for all three surfaces sputtered, and over a wide range of operating conditions. This strongly indicates that the intrinsic structure of the ion-induced surface does not play a significant role in the formation of sputtered cluster anions. The observed abundance patterns of Cn - and CnN- clusters also suggest that mono-nitrogen substitution of the carbon clusters does not allow the chain to ring transformation to occur up to n = 10.

Studies of Linear CnSe- (1 ≤ n ≤ 11) Clusters Produced from Laser Ablation: Collision-Induced Dissociation and ab Initio Calculations

Small carbon cluster anions doped with single selenium atom CnSe- (n = 1 to 11) were produced by laser ablating the mixture of selenium and carbon powders and were recorded by mass spectrometry. Distribution of the doped cluster ions was found to vary with the mixed ratios of the sample, but only the cluster with even number of carbon atoms could be produced in the experiment. Experiment of collision-induced dissociation (CID) verified molecular formulas of the selenium polycarbon clusters and found that they tend to lose a Se atom (for smaller size) or a CSe unit (for larger size). To examine the odd/even alternation effect and other structural features, the cluster ions were further investigated by ab initio calculations with ROHF and B3LYP methods using 6-31G* basis set. Based on the studies of other CnX- clusters and statistical distribution of the CnSe- clusters, geometry of the clusters was assumed to be linear chain with the selenium atom locating at its terminal and was fully optimized in the calc...

Electronic absorption and resonance Raman spectra of large linear carbon clusters isolated in solid argon

Neutral and anionic carbon clusters have been generated via a laser-induced graphite-based plasma and deposited in a solid argon matrix. Anionic clusters were formed from neutral clusters by using crossed electron/carbon cluster beams. Thermal annealing (to 36 K) resulted in the aggregation of the smaller carbon species, leading to the formation of long chain neutral and anionic clusters. Spectroscopic measurements in the ultraviolet, visible, near-infrared and infrared regions revealed a series of bands attributable to a homologous set of odd-numbered C 5–C 29 neutral clusters and even-numbered C 6 −–C 36 − anionic clusters. Good agreement is found for the band positions of carbon chains containing odd C 15–C 21 neutrals and even C 6 −–C 22 − anions, with species previously identified by Maier and coworkers using mass selection or laser vaporization, followed by neon matrix isolation. Resonance Raman frequencies for the neutral C 17, C 21 and C 23 species are shown to be consistent with the above attributions. Density functional theory calculations agree well with the observed bands. It is found that certain low frequency Raman stretching frequencies decrease in a predictable way with increasing chain length. Comparison of the 0 0 0 absorption transitions of the even C 18 −–C 36 − anionic clusters with the ‘unidentified’ infrared (UIR) interstellar emission bands suggests that the electronic emission from specific long chain carbon anions may contribute to the some of the UIR bands.

Photoelectron spectroscopy study of MC− (M=Sc, Y, and La, 5≤n≤20)

Abstract Photoelectron spectra of metal-attached carbon cluster anions MC n − (M=Sc, Y, and La, 5≤ n ≤20) generated in a laser-vaporization ion source were measured using a magnetic bottle time-of-flight photoelectron spectrometer with 6.4 eV photon energy. The vertical detachment energy (vDE) of each species was estimated from the spectra and compared with the vDE of the linear and monocyclic ring forms of C n . It was found that PES features of MC n − (5≤ n ≤20, n =odd) could be understood under the assumption that the π electronic structure of MC n − is similar to those of the linear carbon chains. However, the PES features of MC n − (12≤ n ≤20, n =even) indicate the presence of a π electronic structure similar to the isomers having ring form. Further possible electronic and molecular structures are presented and discussed for MC n − of odd n and even n , respectively.

Study of the decay of refractory clusters by photoelectron imaging spectroscopy

Single-photon photodetachment of mass-selected tungsten and carbon cluster anions has been studied by photoelectron imaging spectroscopy. Velocity map imaging allows us to measure simultaneously the kinetic energy spectrum and the angular distribution of photoelectrons. This provides a clear distinction between the two major decay mechanisms: the isotropic thermionic emission and the anisotropic direct photoemission. A careful study of threshold electrons shows that the thermal distribution cannot be described, even qualitatively, by a simple exponentially decreasing Boltzmann function. Based on qualitative arguments and on previous theoretical works describing thermionic emission in small metallic spheres, the kinetic energy distribution of electrons corresponding to thermionic emission in tungsten cluster anions is found to vary as p(∈) ∈ 1/2 exp(-∈/k B T), in contrast with the bulk-like emission profile p(∈) ∈ exp(-∈/k B T). On the contrary, in negative carbon clusters that do not exhibit spherical symmetry at small size, this simple model cannot describe the thermal electron spectrum. Moreover, in tungsten clusters, the asymmetry parameter β of the most intense band observed in direct photoemission decreases monotonically with size. This probably indicates the loss of coherence induced by electron-electron collisions occurring in large systems prior to electron-phonon coupling.

Two Types of Mass Abundance Distributions for Anionic Carbon Clusters Investigated by Laser Vaporization and Pulsed Molecular Beam Techniques

Two types of mass spectra for anionic carbon clusters Cn- have been revealed using laser vaporization and pulsed molecular beam techniques. The less structured mass spectrum characteristic of the magic-numbers at n = 5, 8, 11, 15, and 17 is established at the early stage of the cluster formation process, namely, in the laser vaporization process. The more structured one is featured for a regular odd-even alternation and the magic numbers at n = 10, 12, 16, 18, 22, and 28, and has been developed only after extensive clustering and quenching processes, where low-energy electron attachment plays a vital role. Transition between these two types of mass spectra can be realized by controlling either the strength of the pulsed gas flow or the synchronism between the gas flow and the laser vaporization.

The interplay between the electronic structure and reactivity of aluminum clusters: model systems as building blocks for cluster assembled materials

Reactions of aluminum–carbon cluster anions with oxygen were investigated under well-defined thermal conditions in a fast flow reactor. The resistance of particular species to the etching of oxygen is observed and is explained through the predicted shell closings of the Jellium model. The presence of Al 7C −, in particular, is compatible with this cluster being a composite Jellium system stabilized by ion-polarizability interactions. This would be the first species of which we are aware that exits as a magic cluster in this context.

Isotopic Infrared Absorption Study of C5-, C7-, and C9- Carbon Cluster Anions in Ar Matrices

The carbon cluster anions C5-, C7-, and C9- have been formed via a dual laser beam technique, trapped in Ar matrixes, and studied by infrared spectroscopy. 13C-Isotopic substitution combined with density functional (B3LYP/6-31G*) calculations of vibrational frequencies and intensities has led to the assignment of asymmetric stretching modes in the three anionic clusters. From the excellent fit of the calculated and experimental singly 13C-substituted cluster bands, a linear geometry for all three clusters has been determined.

Observation of New Ring Isomers for Carbon Cluster Anions

High-resolution ion-mobility measurements have resolved a number of isomers for carbon cluster anions in the drift-time region previously assigned to monocyclic rings. Tight-binding calculations have been performed to generate feasible geometries, which were compared to the experimental data by calculating their mobilities. Structures composed of a short chain of carbon atoms attached to a carbon ring are found to be the most plausible geometries for these new isomers. A number of different bicyclic ring isomers have also been resolved for the first time.

Linear aluminum- and phosphorus-doped carbon cluster anions: Mass distribution and ab initio calculations

Both aluminum- and phosphorus-doped carbon cluster anions were produced by laser ablation on mixed powder samples. Dramatic even/odd alternation of the cluster ion signal intensity can be observed in their recorded mass spectra. The mass distributions reveal that the C n Al − clusters with even n are more stable than those with odd n, while the C n P − clusters with odd n are more stable than those with even n. The contrary parity effects are confirmed by ab initio Hartree–Fock calculations carried out on a linear cluster model. Calculated energies, which are expressed as the energy difference between the adjacent clusters as well as the binding energy and the electron binding energy, and optimized geometries of the clusters also favor the singlet species, which are the C n Al − clusters with even n and the C n P − clusters with odd n.

C3- Carbon Cluster Anion: Structure and Asymmetric Stretching Mode Frequency

Carbon cluster anions have been formed by a combination laser ablation/plasma generation method and then deposited in argon matrices and studied via Fourier transform infrared spectroscopy. In this paper a full isotopic study of the C3- anionic cluster is reported. All six isotopomeric bands of the ν3 antisymmetric stretching mode at 1721.8 cm-1 (all 12C isotopomer) have been observed and used together with a normal coordinate calculation to deduce that C3- in Ar matrices is linear. This is in agreement with previous high-level ab initio calculations and with new density functional theory (DFT) and ab initio (MP2) results. Frequency shifts for all six C3- isotopomers calculated by DFT and MP2 approaches match the observed shifts very well. Finally, evidence is presented that the C3- anionic clusters are formed by electron capture and not by fragment aggregation.

Vibrational Spectroscopy of Small Matrix-Isolated Linear Carbon Cluster Anions

Carbon cluster anions have been generated, deposited in an argon matrix, and studied spectroscopically. A new method involving a dual laser beam-induced Ar plasma using Y or W metal and graphite targets is introduced for the efficient production of matrix-isolated carbon cluster anions. New bands found in the infrared spectra of the deposited plasma mixture have been assigned to asymmetric stretching modes of the C3, C5, C6, C7, and C9 anions. Density functional theory calculations (B3LYP/6-31G* level) support the attribution of these bands to linear carbon cluster anions. Calculational results also indicate that reactions between linear anionic clusters and linear neutral clusters are highly exothermic and that aggregation between anions and even-membered neutrals are more highly exothermic than between anions and odd-membered neutrals. The anion infrared bands are found to be photosensitive at irradiation wavelengths shorter than a certain threshold value. These values are ca. 0.5 eV higher than the cor...