Temporal pulsewidth and the wavelength dependences of the product ions obtained by laser ablation of solid C<Subscript>60</Subscript>

By ablating solid C₆₀ with a laser pulse, we observe various processes such as the prompt- and the delayed-ionization of C₆₀, the fragmentation into molecular ions and the formation of cluster ions. We found these processes show distinct dependences on the temporal pulse width, the power and the wavelength of the ablation laser. From the observations, we could confirm efficient coupling of laser energy to C₆₀ through the molecular absorption even with a laser pulse width less than the electron-phonon coupling time of the C₆₀ molecule.     

Investigations on the behaviour of C60 as a resist in X-ray lithography

Oxygen-free C₆₀ films on various substrates have been used as negative resist in X-ray lithography, yielding pure-carbon microstructures of good quality via synchrotron irradiation through an X-ray mask and subsequent development. While X-ray s induce the polymerization of C₆₀ into insoluble product, large numbers of secondary electrons backscattered from the substrate inhibit this, which we attribute to the different cross sections of X-ray s and electrons in C₆₀. Both electrons and X-ray s generate neutral electronically excited C₆₀ molecules. At a low density of excited C₆₀, as generated by X-ray s, they react predominantly with neighbouring molecules in their ground state via a 2+2-cycloaddition similar to UV polymerization. At a high density of excited C₆₀, as generated by secondary electrons, the excited molecules are not able to react with each other due to orbital symmetry. Instead, the excited states quench each other, thus inhibiting the polymerization. The reduction of the resolution in the C₆₀ pattern, and the inhibition of the polymerization near the interface through backscattered electrons, can be reduced by using substrate materials from which only few electrons emerge. To maintain the density of excited C₆₀ molecules at the interface below the point where the quenching reaction prevails, low synchrotron-radiation intensities are recommended. PACS 81.05.Tp; 82.50.-m; 85.40.Hp     

Electronic interactions in two-dimensional polymers of the C60 fullerene

The electronic structure of tetragonal and rhombohedral polymers of the C₆₀ fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C₆₀ molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C₆₀ fullerene polymers leads to a broadening of the maxima in the CK _α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C₆₀ fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C₆₀ molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level.     

Functionalization of pentagon–pentagon edges of fullerenes by cyclic polysulfides: A DFT study

We have performed a computational study to investigate the cyclosulfurization of the pentagon–pentagon (p–p) junctions in the non-IPR fullerenes C₆₀(D₃) and C₇₀(C_2v), and also Stone-Wales defective C₆₀ fullerene. Our results indicate the exothermic character of cyclosulfurization processes which can be related to the increase of pyramidalization angle (spherical excesses) and p characters of natural hybrid orbitals of C atoms at the p–p junctions. In fact these lead to the structural strain relief and stability of the cyclosulfurization derivatives of the non-IPR fullerenes. Moreover, the cyclosulfurization reaction of p–p bonds on the C₇₀(C_2v) is more energetically favorable than that of C₆₀(D₃), due to the higher curvature of carbon sites and the larger values of the p characters of natural hybrid orbitals in the C₇₀(C_2v). On the other hand, localization of the excess electrons on the C atoms at the p–p junctions leads to the low tendency of the charged non-IPR fullerenes to cyclosulfurization process. The desulfurization pathway of the exohedral derivatives of C₇₀(C_2v) indicates that it is energetically unfavorable for the functionalized fullerenes to break into individual fullerene and sulfur molecules. HOMO–LUMO gaps almost are independent of the number of pentathiepin rings while sensitive to the type of parent fullerene.     

Processing of C<Subscript>60</Subscript> thin films by Matrix-Assisted Pulsed Laser Evaporation (MAPLE)

Thin films of fullerenes (C₆₀) were deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out from a frozen homogeneous dilute solution of C₆₀ in anisole (0.67 wt%), and over a broad range of laser fluences, from 0.15 J/cm² up to 3.9 J/cm². MAPLE has been applied for deposition of fullerenes for the first time and we have studied the growth of thin films of solid C₆₀. The fragmentation of C₆₀ fullerene molecules induced by ns ablation in vacuum of a frozen anisole target with C₆₀ was investigated by matrix-assisted laser desorption/ionization (MALDI). Our findings show that intact fullerene films can be produced with laser fluences ranging from 0.15 J/cm² up to 1.5 J/cm².     

Chemically induced dynamic electron polarization of C60 nitroxide bisadducts and spin multiplicity of their excited states

A series of biradicals consisting of a C₆₀ linked to two nitroxide addends have been studied by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The series includes all four trans bisadducts and the equatorial isomer. After a visible laser pulse, the bisadducts in toluene liquid solution show chemically induced dynamic electron polarization (CIDEP) effects with a complex pattern of narrow lines in emission and enhanced absorption. From the knowledge of the sign of exchange interaction between the two nitroxide moieties, it was possible to account for the CIDEP effect in terms of an intramolecular triplet-triplet annihilation. A broad signal attributed to the quintet excited state was observed at short time delay after the laser pulse. The TR-EPR spectra recorded at low temperature in glassy matrix are assigned to an excited triplet state localized on the C₆₀, with a small interaction with the spins of the two nitroxide fragments. The assignment is based on spectral simulation and analysis of the transient nutation frequency. The small exchange interaction between excited triplet C₆₀ and nitroxide is shown to depend on the nature of the nitroxide addend.     

Slow decay of negative molecular fluorofullerene ions in the electron autodetachment process

The mean lifetimes of negative molecular ions of C₆₀ fullerene and its fluoroderivatives C₆₀F₁₈ and C₆₀F₃₆ with respect to the autodetachment of electrons have been measured as functions of the ionizing electron energy by the method of mass spectrometry of electron resonance capture. The lifetimes of negative ions in the compounds under investigation are within the one-second range, and an increase in the number of addends leads to an increase in the lifetimes of negative ions by 1.5–2.5 orders of magnitude.     

Molecular fusion within fullerene clusters induced by femtosecond laser excitation

Molecular fusion is induced in clusters of fullerene molecules on excitation with fs laser pulses. The dependence of the mass distributions of the fused products on the initial cluster distribution are studied and results for (C₆₀)_N and (C₇₀)_N clusters are compared. The fused products decay by emitting C₂ molecules and the fragmentation spectrum is used to determine the initial excitation energy of the fused species. The threshold excitation energy needed to induce fusion is consistent with the energetic thresholds for molecular fusion of fullerenes determined previously in single collision experiments.     

Theory for the ultrafast dynamics of excited clusters: interplay between elementary excitations and atomic structure

We present a theoretical study of the short-time relaxation of clusters in response to ultrafast excitations using femtosecond laser pulses. We analyze the excitation of different types of clusters (Hg_n, Ag_n, Si_n, C₆₀ and Xe_n) and classify the relaxation dynamics in three different regimes, depending on the intensity of the exciting laser pulse. For low-intensity pulses (I<10¹² W/cm²) we determine the time-dependent structural changes of clusters upon ultrashort ionization and photodetachment. We also study the laser-induced non-equilibrium fragmentation and melting of Si_n and C₆₀ clusters, which occurs for moderate laser intensities, as a function of the pulse duration and energy. As an example for the case of high intensities (I>10¹⁵ W/cm²), the explosion of clusters under the action of very intense ultrashort laser fields is described. Received: 26 November 1999 / Published online: 2 August 2000     

Structure and electronic properties of Hn@C20 molecule

Density functional theory has been employed to optimize the structure of endohedral doped C₂₀ fullerene. We have also investigated electronic properties. We have found that C₂₀ cage can accommodate up to 8 hydrogen atoms. Some hydrogen atoms get chemisorbed on the inner surface of C₂₀ cage and form C-H bond. Structural deformation is found to increase with increase in H-atoms. From the analysis of electronic properties, we observe that due to endohedral doping of hydrogen atoms inside C₂₀, H-atoms acquire net negative charge by accepting electrons and fullerene molecules acquire positive charge by donating electrons to H-atoms. For endohedral complexes where H₃ triangular molecule formation takes place, the nature of net charge transfer changes, i.e. fractional electronic charge is transferred from H-atoms to fullerene. C₂₀ doped with odd number of H-atoms should be more reactive compared to the even number case. Most of the present results are similar to those of endohedral C₆₀.     

Cluster–cluster fusion

An experimental study of molecular fusion in fullerene–fullerene collisions is presented and the theoretical interpretation of the cross section is reconsidered in terms of phase space arguments and competition with direct collision induced dissociation. The form and absolute magnitude of the cross sections for C⁺₆₀+C₇₀ (or C⁺₇₀+C₆₀) and C⁺₇₀+C₇₀ can be understood, however, the much smaller cross section for C⁺₆₀+C₆₀ remains a puzzle. The fragmentation behaviour of the hot fusion product is well described by a maximal entropy model indicating equipartition of the centre of mass collision energy followed by statistical fragmentation. To cite this article: E.E.B. Campbell et al., C. R. Physique 3 (2002) 341–352.     

C2和C2+,C2-的电子结构与势形共振

本文在独立电子近似的基础上,根据多重散射自洽场理论方法,计算了C₂和C₂⁺,C₂^-分子(离子)的电子结构,阐明了势形共振能量和上述分子(离子)电子数的关系,结果表明,随着电子数的减少,C原子2s-2p轨道杂化减弱,势形共振的能量将降低(如降低到阈值下,则势形共振消失)。 关键词：     

TR-EPR of single and double spin-labeled C60 derivatives in frozen matrices

Time-resolved electron paramagnetic resonance spectra of two single spin-labeled and two double spin-labeled C₆₀ derivatives in frozen solution are recorded with pulsed laser excitation and 100 ns time resolution. Quartet and quintet excited species are detected which arise from the electron spin coupling of the triplet excited fullerene moiety with the unpaired spin(s) of the nitroxide label(s). Despite the similar molecular structure, in both series of single and double labeled derivatives a different behavior was found, which is due to substantial difference of the energy of exchange coupling.     

Molecular dynamics simulation of the low-energy interaction between Cu<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>@C<Subscript>60</Subscript> endofullerenes and the surface of a copper crystal

A molecular dynamics simulation of the low-energy interaction of C₆₀ fullerenes and Cu₁@C₆₀, Cu₆@C₆₀, and Cu₁₃@C₆₀ endofullerenes with a Cu(100) surface was performed. The effects of a copper cluster encapsulated in a fullerene and of a fullerene’s translational motion and rotation energy on its penetration into a surface were investigated. It was shown that the presence of an encapsulated cluster has a positive effect on fullerene penetration into a surface with preservation of the fullerene’s structure. The optimal conditions for fullerene penetration into a copper crystal surface were determined.     

Electron multiplicity in slow collisions of Ar ions with C

Electron capture by Ar⁸⁺ in collisions with C₆₀ fullerene has been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of multicharged Cr+ ₆₀ recoil ions and their fragments Ci+ m and the final charge state of outgoing projectiles Ar(8-s)+ (). The number of captured electrons r is the sum of the numbers of stabilized and emitted electrons: r = n + s. The ratio n / s decreases by a factor three with s increasing from 1 to 7 showing that the multiply excited states populated by capture of a large number of electrons are rather stable against auto-ionisation. Each kinetic energy spectrum of Ar⁺ and Ar²⁺ projectiles is composed of two peaks which we attribute to collisions “inside” and “outside” the C₆₀ cage. The measured energy shift of the projectile keV is consistent with the corresponding energy loss keV in a carbon foil with an equivalent thickness. Inside collisions are characterized by a strong dissociation of recoil ions into light monocharged fragments and by a high multiplicity of ejected electrons. Received: 25 March 1998 / Received in final form and Accepted: 9 June 1998     

New property of the fullerenes: the anomalously effective quenching of electronically excited states owing to energy transfer to the C<Subscript>70</Subscript> and C<Subscript>60</Subscript> molecules

The quenching of the electronically excited states of various energy donors—Tb³⁺; 9,10-anthracene dibromide; and adamantanone—by C₇₀ fullerene has been detected and analyzed. The quenching is characterized by anomalously high biomolecular quenching rate constants, which are obtained from the Stern-Volmer dependences of the energy-donor photoluminescence intensity on the concentration of the C₇₀ molecules. It has been shown that the high efficiency of quenching by the C₇₀ fullerene as compared to the C₆₀ fullerene is due to the higher polarizability of the C₇₀ molecule and large overlap integrals of the energy-donor photoluminescence spectra with the absorption spectrum of the C₇₀ fullerene.     

The compressive mechanical properties of Cn (n=20, 60, 80, 180) and endohedral M@C60 (M=Na,Al, Fe) fullerene molecules

The compressive mechanical properties of C_n (n = 20, 60, 80, 180) and endohedral M@C₆₀ (M = Na, Al, Fe) fullerene molecules are investigated using a quantum molecular dynamics (QMD) technique. Energy–strain curves, force–strain curves, endurance load, failure strain corresponding to the endurance load, and compressive stiffness of the fullerene molecules are obtained. The compressive mechanical properties of C₂₀, C₆₀, C₈₀, C₁₈₀ and M@C₆₀ (M = Na, Al, Fe) are discussed. The results show that the larger the magic number n of an empty fullerene, the higher its endurance load and compressive stiffness, but the lower its failure strain, and comparing to the empty C₆₀ fullerene, all the M@C60 molecules have greater endurance capability and failure strain.     

Creation of a donor-acceptor pair for studying intraprotein electron transfer with the participation of amino-acid derivatives of fullerene C60

Efficient quenching of eosin phosphorescence by amino-acid derivatives of fullerene (AADFs) such as C₆₀-alanine and C₆₀-glycine in aqueous solutions indicates the possibility of transferring electrons from eosin to fullerene upon collisions or in the exciplex state. To investigate electron transfer in the protein structure, we studied the process of incorporation of C₆₀-alanine and C₆₀-glycine into the heme pocket of myoglobin by controlling Förster quenching. The dissociation constant for the protein-AADF complex was estimated.     

Optical limiting with higher fullerenes

Optical limiting has been investigated for higher fullerenes and compared with C₆₀. The transmission through an aperture placed after solutions of C₇₆, C₇₈, and C₈₄ in tetrahydronaphthalene was measured using Q-switched laser pulses with a wavelength of 532 nm and a pulse width of 8 ns FWHM. Unlike C₆₀, the transmission for these higher fullerene solutions decreased linearly with increasing optical pulse energy. We attribute the linearized optical limiting response to self-defocusing of the optical beam and the absence of excited-state absorption. The ground state absorption spectra for the higher fullerenes suggest their use for optical limiting in the near infrared, and the C₈₄-tetrahydronaphthalene solution was found to be an optical limiter at 1.064 m.