Coulomb stability limit of highly charged Cq+60 fullerenes

On the thermodynamic stability of polycations

We present a simple approximation to estimate the largest charge that a given molecule can hold until fragmentation into smaller charged species becomes more energetically favorable. This approximation solely relies on the ionization potentials, electron affinities of the parent and fragment species, and also on the neutral parent's dissociation energy. By parameterizing these quantities, it is possible to obtain analytical phase diagrams of polycationic stability. We demonstrate the applicability of this approach by discussing the maximal charge dependence on the size of the molecular system. A numerical demonstration for linear polyenes, monocyclic annulenes, and helium clusters is provided.

High-Stability Light-Element Magnetic Superatoms Determined by Hund's Rule

Achieving stable high-magnetism light-element structures at nanoscale is vital to the field of magnetism, which has traditionally been ruled by transition-metal elements with localized d or f electrons. By first-principles calculations, we show that superatoms made of pure earth-abundant light elements (i.e., boron and nitrogen) exhibit desired magnetic properties that rival those of rare-earth elements, and the magnetism is dictated entirely by Hund's maximum spin rule. Importantly, the chemical and structural stabilities of the superatoms are not jeopardized by its high spins and are in fact better than those of transition-metal-element-embedded clusters. Our work thus establishes the basic principles for designing novel light-element, high-stability, and high-moment magnetic superatoms.

Unraveling the Metastability of C n2+ ( n = 2-4) Clusters

Pure carbon clusters have received considerable attention for a long time. However, fundamental questions, such as what the smallest stable carbon cluster dication is, remain unclear. We investigated the stability and fragmentation behavior of C _n²⁺ ( n = 2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to C _n-1⁺ + C⁺. During C₂²⁺ → C⁺ + C⁺, significant kinetic energy release (∼5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic ¹²C₂²⁺ dications are either in weakly bound ³Π_u and ³Σ_g^- states, quickly dissociating under the intense electric field, or in a deeply bound electronic ⁵Σ_u^- state with lifetimes >180 ps.

Adventures in boron chemistry - the prediction of novel ultra-flexible boron oxide frameworks

Recent periodic density functional calculations have predicted the existence of ultra-flexible low-energy forms of boron oxides in which rigid boron-oxygen heterocycles are linked by flexible B-O-B bridges. The minima in the energy landscapes of these frameworks are remarkably broad, with widths in excess of those of many hybrid metal-organic frameworks. Enormous changes in cell volume, which can exceed a factor of two, are accompanied by negligible changes in energy. Here we explore the underlying reasons for this behaviour using molecular electronic-structure calculations, periodic density functional theory and template-based geometric simulations. The angular flexibility of the B-O-B bridge depends only upon the geometry of the local B2O5 unit, independent of the configuration of neighbouring bridges. Unique cooperativity between the bending and twisting motions of the bridges leads to considerable anisotropy in framework flexibility. Exceptional flexibility is conferred not only by the intrinsic bending flexibility of the bridges but by topological factors, crucially the relaxation of torsional constraints when B3O3 rings are present. We test these conclusions by showing how the flexibility of the frameworks can be tuned by decoration with isoelectronic rings. The new nanoporous boron oxides presented in this work are predicted to be potential novel guest-host materials because of their flat energy landscapes. Furthermore, such structures can be generated systematically from silicates by the substitution of B2O54- for SiO44-. A borate analogue of β-cristobalite is shown to be isoenergetic with the known B2O3-I polymorph. We raise the possibility of new families of frameworks and zeolite analogues.

Absolute fragmentation cross sections in atom-molecule collisions: scaling laws for non-statistical fragmentation of polycyclic aromatic hydrocarbon molecules

We present scaling laws for absolute cross sections for non-statistical fragmentation in collisions between Polycyclic Aromatic Hydrocarbons (PAH/PAH(+)) and hydrogen or helium atoms with kinetic energies ranging from 50 eV to 10 keV. Further, we calculate the total fragmentation cross sections (including statistical fragmentation) for 110 eV PAH/PAH(+) + He collisions, and show that they compare well with experimental results. We demonstrate that non-statistical fragmentation becomes dominant for large PAHs and that it yields highly reactive fragments forming strong covalent bonds with atoms (H and N) and molecules (C6H5). Thus nonstatistical fragmentation may be an effective initial step in the formation of, e.g., Polycyclic Aromatic Nitrogen Heterocycles (PANHs). This relates to recent discussions on the evolution of PAHNs in space and the reactivities of defect graphene structures.

Thiol-based molecular overlayers adsorbed on C60: role of the end-group and charge state on the stability of the complexes

We present pseudo-potential density functional theory calculations dedicated to analyze the stability and electronic properties of thiol-based molecular overlayers adsorbed on C60. We consider short molecules having a S atom as a headgroup, alkyl chains containing one to three C atoms, and a CH3 species as a terminal group. The thiol molecules are bonded to the carbon surface (through the S atom) with adsorption energies that vary in the range of ~1-2 eV and with S-C bond lengths of ~1.8 Å. For neutral C60(SCH3)n complexes, low energy atomic configurations are obtained when the thiol groups are distributed on the surface forming small molecular domains (e.g., pairs, trimers, or tetramer configurations of neighboring thiol molecules). In contrast, less stable random distributions are defined by orientationally disordered overlayers with highly distorted underlying carbon networks. The inclusion of London dispersion interaction slightly affects the structure of the molecular coating but increases the adsorption energies by values as large as 0.3 eV. Interestingly, the relative stability of the previous adsorbed phases differ from the one obtained when considering single sulfur adsorption on C60, a result that reveals the crucial role played by the terminal CH3 groups on the structure of the molecular coating. The positive (negative) charging of the [C60(SCH3)n](±q) complexes, with q as large as 8e, changes the geometrical structure and the chemical nature of the ligand shell inducing lateral molecular displacements, S-S bonding between neighboring thiols, as well as the partial degradation of the molecular coating. Finally, we consider the stability of two-component mixed overlayers formed by the coadsorption of CH3-, OH-, and NH2-terminated alkanethiols of the same length. In agreement with the results found on Au surfaces, we obtain lowest energy atomic configurations when molecular domains of a single component are stabilized on C60, a result that could be of fundamental importance in biomedical applications.

Scaling law for the partitioning of energy in fragmenting multicharged carbon clusters

The complete fragmentation of highly excited and multicharged C(n)(q+) clusters (n=5-10; q=2-4), produced in high velocity collisions of C(n)(+) with atoms, has been measured. Multiplicity distributions are presented and used to deduce, within a statistical framework, the partitioning of energy between the fragments' production and fragments' kinetic energy. This partitioning is found to scale as the charge over mass ratio of the cluster.

Density functional theory study of multiply ionized weakly bound fullerene dimers

Multiply ionized fullerene dimers ([C(60)](2) (q+),q=1-6) have been studied by means of state-of-the-art density functional theory methods. We found that the singly charged dimer is more strongly bound than the corresponding neutral van der Waals dimer at the binding distance of the latter; in contrast, multiply charged dimers (q>or=2) are unstable. For the latter dimers, the calculated kinetic energy release in the binary fission yielding intact fullerenes is lower than those reported in recent experimental work. This implies that, in such experiments, there are significant internal excitations in the separating monomers. We also show that electron transfer within the charged dimers occurs on the subfemtosecond time scale, in accordance with the high charge mobility observed in dimers and larger clusters of fullerenes. This provides an explanation for the even-odd effects in the measured multiple ionization yields of fullerene dimers.

Absolute charge transfer and fragmentation cross sections in He2+-C60 collisions

We have determined absolute charge transfer and fragmentation cross sections in He2++C60 collisions in the impact-energy range 0.1-250 keV by using a combined experimental and theoretical approach. We have found that the cross sections for the formation of He+ and He0 are comparable in magnitude, which cannot be explained by the sole contribution of pure single and double electron capture but also by contribution of transfer-ionization processes that are important even at low impact energies. The results show that multifragmentation is important only at impact energies larger than 40 keV; at lower energies, sequential C2 evaporation is the dominant process.

Theoretical study of the stability of multiply charged C70 fullerenes

We have calculated the electronic energies and optimum geometries of C(70) (q+) and C(68) (q+) fullerenes (q=0-14) by means of density functional theory. The ionization energies for C(70) and C(68) fullerenes increase more or less linearly as functions of charge, consistent with the previously reported behavior for C(60) and C(58) [S. Diaz-Tendero et al., J. Chem. Phys. 123, 184306 (2005)]. The dissociation energies corresponding to the C(70) (q+)-->C(68) (q+)+C(2), C(70) (q+)-->C(68) ((q-1)+)+C(2) (+), C(70) (q+)-->C(68) ((q-2)+)+C(+)+C(+), C(70) (q+)-->C(68) ((q-3)+)+C(2+)+C(+), and C(70) (q+)-->C(68) ((q-4)+)+C(2+)+C(2+) decay channels show that C(70) (q+) (like C(60) (q+)) is thermodynamically unstable for q>or=6. However, the slope of the dissociation energy as a function of charge for a given decay channel is different from that of C(60) (q+) fullerenes. On the basis of these results, we predict q=17 to be the highest charge state for which a fission barrier exists for C(70) (q+).

Stability of highly OH-covered C60 fullerenes: role of coadsorbed O impurities and of the charge state of the cage in the formation of carbon-opened structures

We have performed both semiempirical as well as ab initio density functional theory calculations in order to investigate the structural stability of highly hydroxylated C60(OH)32 fullerenes, so-called fullerenols. Interestingly, we have found that low-energy atomic configurations are obtained when the OH groups are covering the C60 in the form of small hydroxyl islands. The previous formation of OH molecular domains on the carbon surface, stabilized by hydrogen bonds between neighboring OH groups, defines the existence of C60(OH)32 fullerene structures with some elongated C-C bonds, closed electronic shells, and large highest occupied-lowest unoccupied molecular orbital energy gaps, with the latter two being well-known indicators of high chemical stability in these kind of carbon compounds. The calculated optical absorption spectra show that the location of the first single dipole-allowed excitation strongly depends on the precise distribution of the OH groups on the surface, a result that, combined with optical spectroscopy experiments, might provide an efficient way to identify the structure of these kinds of fullerene derivatives. We found that the presence of a few coadsorbed oxygen species on the fullerene surface leads in general to the existence of C60(OH)32O(x) (x = 1-4) compounds in which some of the C-C bonds just below the O impurities are replaced by C-O-C bridge bonds, leading to the formation of stable carbon-opened structures in agreement with the recent experimental work of Xing et al. (J. Phys. Chem. B 2004, 108, 11473). Actually, a more dramatic cage destruction is obtained when considering multiply charged C60(OH)32O(x)(+/-m) (m = 2, 4, 6) species (that can exist in both gas-phase and aqueous environments), where now sizable holes made of 9- and 10-membered rings can exist in the carbon network. We believe that our results are important if the controlled opening of carbon cages is needed and it should be taken into account also in several technological applications where the permanent encapsulation of atomic or molecular species in these types of fullerene derivatives is required.

Stabilities of multiply charged dimers and clusters of fullerenes

The authors find even-odd variations as functions of r (<or=7) for multiple ionization of van der Waals dimers in slow Xe(30+)+[C60]2([C60C70])-->...+[C60]2(r+)([C60C70](r+)) electron-transfer collisions. This even-odd behavior is in sharp contrast to the smooth one for fullerene monomers and may be related to even-odd effects in dimer ionization energies in agreement with results from an electrostatic model. The kinetic energy releases for dimer dissociations [predominantly yielding intact fullerenes [C60]2(r+)-->C60(r1+)+C60(r2+) in the same (r1=r2) or nearby (r1=r2+/-1) charge states] are found to be low in comparison with the corresponding model results indicating that internal excitations of the separating (intact) fullerenes are important. Experimental appearance sizes for the heavier clusters of fullerenes [C60]n(r+) (n>3 and r=2-5) compare well with predictions from a new nearest-neighbor model assuming that r unit charges in [C60]n(r+) are localized to r C60 molecules such that the Coulomb energy of the system is minimized. The system is then taken to be stable if (i) two (singly) charged C60 are not nearest neighbors and (ii) the r C60(+) molecules have binding energies to their neutral nearest neighbors which are larger than the repulsive energies for the (r-1) C60(+)-C60(+) pairs. Essential ingredients in the nearest-neighbor model are cluster geometries and the present results on dimer stabilities.

Single-active-electron ionization of C60 in intense laser pulses to high charge states

Sequential ionization of the C(60) fullerene to high charge states in ultrashort intense laser pulses is investigated within the strong-field S-matrix approach. Ion yields are calculated and saturation intensities are determined for a broad range of laser wavelengths between 395 and 1800 nm at different pulse lengths. Comparisons of the S-matrix predictions for the saturation intensities with recent experimental data are in an overall satisfactory agreement, indicating that saturation of ionization of this complex molecule can be well described using the single-active-electron approach. The analysis of the results shows that the contributions from the h(u)-highest occupied molecular orbital to the ion yields dominate as compared to those from the inner valence shells h(g) and g(g). Finally, it is demonstrated that the suppression of ionization of C(60) and its ions, as observed in experiments, can be interpreted within the present theory as due to the finite cage size of the fullerenes and a multi-slit-like interference effect between partial waves emitted from the different nuclei of the fullerenes.

C60 in intense short pulse laser fields down to 9fs: Excitation on time scales below e-e and e-phonon coupling

The interaction of C60 fullerenes with 765-797 nm laser pulses as short as 9 fs at intensities of up to 3.7 x 10(14) W cm(-2) is investigated with photoion spectroscopy. The excitation time thus addressed lies well below the characteristic time scales for electron-electron and electron-phonon couplings. Thus, energy deposition into the system is separated from energy redistribution among the various electronic and nuclear degrees of freedom. Insight into fundamental photoinduced processes such as ionization and fragmentation is obtained from the analysis of the resulting mass spectra as a function of pulse duration, laser intensity, and time delay between pump and probe pulses, the latter revealing a memory effect for storing electronic energy in the system with a relaxation time of about 50 fs. Saturation intensities and relative abundances of (multiply charged) parent and fragment ions (C60(q+), q=1-6) are fingerprints for the ionization and fragmentation mechanisms. The observations indicate that for final charge states q>1 the well known C60 giant plasmon resonance is involved in creating ions and a significant amount of large fragments even with 9 fs pulses through a nonadiabatic multielectron dynamics. In contrast, for energetic reasons singly charged ions are generated by an essentially adiabatic single active electron mechanism and negligible fragmentation is found when 9 fs pulses are used. These findings promise to unravel a long standing puzzle in understanding C60 mass spectra generated by intense femtosecond laser pulses.

Theoretical investigation of the stability of highly charged C60 molecules produced with intense near-infrared laser pulses

We theoretically investigated the stability of highly charged C(60) (z+) cations produced from C(60) with an ultrashort intense laser pulse of lambda approximately 1800 nm. We first calculated the equilibrium structures and vibrational frequencies of C(60) (z+) as well as C(60). We then calculated key energies relevant to dissociation of C(60) (z+), such as the excess vibrational energy acquired upon sudden tunnel ionization from C(60). By comparing the magnitudes of the calculated energies, we found that C(60) (z+) cations up to z approximately 12 can be produced as a stable or quasistable (microsecond-order lifetime) intact parent cation, in agreement with the recent experimental report by V. R. Bhardwaj et al. [Phys. Rev. Lett. 93, 043001 (2004)] that almost only intact parent C(60) (z+) cations up to z=12 are detected by a mass spectrometer. The results of Rice-Ramsperger-Kassel-Marcus calculation suggest that the lifetime of C(60) (z+) drastically decreases by ten orders of magnitude as z increases from z=11 to z=13. Using the time-dependent adiabatic state approach, we also investigated the vibrational excitation of C(60) and C(60) (z+) by an intense near-infrared pulse. The results indicate that large-amplitude vibration with energy of >10 eV is induced in the delocalized h(g)(1)-like mode of C(60) (z+).

Structure and electronic properties of highly charged C60 and C58 fullerenes

We present a theoretical study of the structure and electronic properties of positively charged C60(q+) and C58(q+) fullerenes (q = 0-14). Electronic energies and optimum geometries have been obtained using density-functional theory with the B3LYP functional for exchange and correlation. We have found that closed- and semiclosed-shell C60(q+) ions (q = 0, 5, and 10) preserve the original icosahedral symmetry of neutral C60. For other charges, significant distortions have been obtained. The C58(q+) fullerenes are, in general, less symmetric, being C58(8+) the closest to the spherical shape. Most C60(q+) fullerenes follow Hund's rule for spin multiplicity, while most C58(q+) fullerenes are more stable with the lowest spin multiplicity. The calculated ionization potentials for both kinds of fullerenes increase almost linearly with charge, except in the vicinity of C60(10+) and C58(8+). We have also explored the region of the potential-energy surface of C60(q+) that leads to asymmetric fission. Minima and transition states corresponding to the last steps of the fission process have been obtained. This has led us to conclude that, for 3 < or = q < or = 8, C2(+) emission is the preferred fragmentation channel, whereas, for higher q values, emission of two charged atomic fragments is more favorable. The corresponding fission barrier vanishes for q > 14.