Ion–Ion Collisions Involving Fullerene Ions

Abstract

Fullerenes are a direct link between atoms with discrete electronic energy levels and solids with a band structure and a well defined surface. In collision experiments, both between energetic ions or electrons and neutral C₆₀ molecules [Walch, B.; Cocke, C.L.; Voelpel, R.; Salzborn, E. Electron capture from C₆₀ by slow multiply charged ions. Phys. Rev. Lett. 1994, 72, 1439–1442; Scheier, P.; Hathiramani, D.; Arnold, W.; Huber, K.; Salzborn, E. Multiple ionization and fragmentation of negatively charged fullerene ions by electron impact. Phys. Rev. Lett. 2000, 84, 55–58; Hathiramani, D.; Aichele, K.; Arnold, W.; Huber, K.; Salzborn, E.; Scheier, P. Electron‐impact induced fragmentation of fullerene ions. Phys. Rev. Lett. 2000, 85, 3604–3607], as well as between charged fullerene ions and neutral targets [Hvelplund, P.; Andersen, L.; Haugen, H.; Lindhard, L.; Lorents, D.C.; Malhotra, R.; Rouff, R. Dynamical fragmentation of C₆₀ ions. Phys. Rev. Lett. 1992, 69, 1915–1918; Hvelplund, P.; Andersen, L.; Brink, C.; Yu, D.; Lorents, D.C.; Rouff, R. Charge transfer in collisions involving multiply charged C₆₀ molecules. Z. Phys. D 1994, 30, 323; Rohmund, F.; Campbell, E.E.B. Resonant and non‐resonant charge transfer in C₆₀ ⁺ + C₆₀ and C₇₀ ⁺ and C₆₀ collisions. J. Phys. B: At. Mol. Opt. Phys. 1997, 30, 5293–5304; Shen, H.; Hvelplund, P.; Mathur, D.; Barany, A.; Cederquist, H.; Selberg, N.; Lorents, D.C. Fullerene–fullerene collisions: fragmentation and electron capture. Phys. Rev. A 1995, 52, 3847–3851], the fundamental processes of electron transfer, ionization, and fragmentation have been studied extensively. Here we report on our experiments on electron transfer in the collision systems C₆₀ ⁺ + ³He²⁺ → C₆₀ ²⁺ + ³He⁺ and C₆₀ ⁺ + C₆₀ ²⁺ → C₆₀ ²⁺ + C₆₀ ⁺. For the latter system we also report on an upper level for the fragmentation probability.     

Dynamics of fragmentation of a multicharge fullerene ion

We have studied the process of fragmentation of a molecular ion of C₆₀ fullerene, which takes place as a result of the loss of five electrons in collision with a 64-keV Ar⁶⁺ ion. The spectrum of kinetic energies of the fragment ions with a given mass, as well as the number of such ions, were determined from the shapes of lines in the mass spectrum measured using a time-of-flight analyzer. It is established that, in addition to the capture of electrons by the projectile ion, an important role in the formation of a multicharge molecular C₆₀ ion is played by additional ionization of the target molecule. It is shown for the first time that the fragmentation of fullerene molecular ions is not a single-stage process. The kinetic energies and the mass spectrum of fragment ions are determined both by the Coulomb repulsion in a multicharge fullerene ion and by the subsequent additional decay of heavy fragment ions in the course of expansion.     

Time-Resolved Mass Spectrometric Studies of Fullerenes and Endohedral Fullerenes

Abstract

The energetics and dynamics of C₂ evaporations from fullerene radical cations C_n ^?+ were studied with and without ion trapping techniques. Electron space charge trapping was applied in electron ionization experiments. RF trapping in a Paul type ion source was applied for Laser MPI. Appearance energies, kinetic energy releases and metastable fractions were determined. the experimental data were modeled by finite heat bath theory (FHBT) and by RRKM/QET. Evaporation (binding) energies as well as conventional and intrinsic kinetic shifts were deduced. Our results support the following conclusions: (a) C₆₀ sits on the leading edge of a magic shell, with C₅₈ only slightly less stable and C₆₂ considerably less stable than C₆₀; (b) Dissociative decay is able to compete with radiative decay of C₆₀ ^?+ only at internal energies in excess of 37.6 eV (T_b>2300 K); (c) a Ne endohedral atom has a slightly stabilizing effect on the C₆₀ ^?+ cage whereas La is strongly stabilizing the C₈₂ ^?+ cage; (d) C₂ loss is a true evaporation in the sense of having a loose transition state with no reverse activation energy; (e) the C₂ evaporations are statistical in nature and theories such RRKM/QET and FHBT are in excellent agreement with experimental observations.     

In-situ analyses on the reactive sputtering process to deposit Al-doped ZnO films using an Al-Zn alloy target

The kinetic energies of generated ions were investigated during the reactive sputtering process to deposit Al-doped ZnO (AZO) films using an Al-Zn alloy target. The sputtering system was equipped with specially designed double feedback system to stabilise the reactive sputtering processes and analysis was performed with a quadrupole mass spectrometer combined with an energy analyser. Negative ions O⁻, O₂⁻, AlO⁻ and AlO₂⁻ with high kinetic energies corresponding to cathode voltage are generated at the partially oxidised target surface, after which some of the ions undergo subsequent charge exchange and/or dissociation. Positive ions O⁺, Ar⁺, Zn⁺ and Al⁺ with lower kinetic energies (around 10 eV) are generated by charge exchange of sputtered neutral O, Ar, Zn and Al atoms, respectively. As the target surface oxidises, cathode voltage decrease, the flux of high-energy negative ions increases and the electrical properties of the AZO degrade by ion bombardment as well as the AZO films that are deposited by conventional magnetron sputtering using an AZO target.     

In-situ analysis of positive and negative energetic ions generated during Sn-doped In2O3 deposition by reactive sputtering

Using a quadrupole mass spectrometer combined with an energy analyser, we have investigated the in-situ energy distribution of highly energetic ions generated during reactive sputtering of In-Sn alloy (IT) targets and non-reactive sputtering of Sn-doped In₂O₃ (ITO) ceramic targets. Ar⁺, In⁺, O⁺, O⁻, O₂⁻, InO⁻ and InO₂⁻ ions with kinetic energies greater than 40 eV were clearly observed. Upon increasing the O₂ flow ratio for reactive sputtering, the surface of the IT target changes from metal (metal mode) to oxide (oxide mode) via a state of mixed metal and oxide (transition region). O⁻ ions with the kinetic energy corresponding to cathode voltage are generated at the oxide layer, which expands upon the target surface with increasing O₂ flow ratio in the metal mode and the transition region. In contrast, the flux of 60-eV Ar⁺ ions decreases with increasing O₂ flow ratio. The presence of 125- and 200-eV In⁺ ions is attributed to the dissociation of InSnO₂⁻ and InO₂⁻ with the kinetic energy corresponding to cathode voltage, respectively, while the presence of 40- and 150-eV O⁺ ions is attributed to the dissociation of InO₂⁻ and O₂⁻ with the kinetic energy corresponding to cathode voltage, respectively.     

Features of the angular distributions of accelerated multiply charged ions in oriented crystals

Features that have been discovered recently in the transmission of multiply charged channeled ions with various energies through crystals are interpreted on the basis of kinetic theory. It is shown that the transition from “cooling” to “heating” of an isotropic beam of heavy multiply charged ions depends not only on the ion energy, but on the depth of their penetration into the crystal as well. The automodel character of the angular distribution of multiply charged ions transmitted through a thin oriented crystal is predicted. Agreement with experiment is achieved on the assumption that, at high ion energies, the probability of electron capture by a channeled ion decreases in comparison to the probability of electron loss. We also discuss the possibility of experimentally measuring the partial angular distributions and the dependence of the electron capture and loss probabilities on the transverse momentum and impact parameter for ions interacting with atomic chains.     

Spontaneous Decay of Highly-Charged Fullerene Ions C60Z+ and C58z+

Abstract

Using isotope-resolved, two sector field mass spectrometric techniques we have identified and investigated quantitatively the energetics and kinetics (in particular the kinetic energy release, KER) of the spontaneous decay reactions C_60–2m ^z+ → C_60-2m-p(z-1)+ ₊ C_p ⁺ with m = 0 or 1, z ranging from 3 to 6 and p = 2 and 4. The obtained KER results are not compatible with the properties expected for a single-step fissioning reaction as described by the liquid drop model. Therefore the present data had to be interpreted by a different fragmentation mechanism. This novel reaction sequence, termed auto charge transfer (ACT) reaction, is initiated by the statistically driven neutral C₂ (or C₄₎ evaporation followed by an electron transfer process from the receding C₂ (or C₄) fragment to the remaining highly-charged fullerene ion thereby leading finally to the two charged reaction products observed in the exit channel of the decay reaction. Moreover, in the case that a C₂ ⁺ loss from C₆₀z+ is occurring in the first field-free region we have been able to demonstrate that it is possible to observe in the second field-free region a subsequent C₂ evaporation from the C₅₈(z-1)⁺ fragment ion.     

A study on the aromaticity and kinetic stability of fullerene C38 isomers and their molecular ions

The aromaticity of the fullerene C₃₈ isomers and their molecular ions is examined by the topological resonance energy (TRE) method. The aromaticity order obtained by the TRE method is compared with relative energies found in DFT calculation results reported in literature. It is found that all C₃₈ isomers exhibit antiaromaticity when in the neutral state. However, they are highly aromatic in the hexaanionic states. The minimum bond resonance energy (min BRE) method is utilized to estimate the kinetically stability of the C₃₈ isomers and their molecular ions. According to the min BRE method, C₃₈ isomers are only have kinetic stability in penta- and hexavalent molecular anionic states.     

Analysis of metallofullerenes using a mass reflectron

A high-resolution mass reflectron has been used to analyze an extract of a fullerene mixture containing the metallofullerene Gd@C₈₂. It is shown that various metallofullerenes can be analyzed by means of thermal desorption followed by electron impact ionization. The C₆₀, C₈₂, and Gd@C₈₂ ion currents are obtained as functions of the evaporation temperature. Doubly charged (Gd@C₈₂)²⁺ metallofullerene ions are identified in the mass spectra. Pis’ma Zh. Tekh. Fiz. 24, 1–5 (March 12, 1998)     

Metastable C3H5 produced by electron impact of propane

Metastability of C₃H₅²⁺ ions formed by electron impact ionization of propane was monitored with the help of a magnetic mass spectrometer of reversed geometry (BEE geometry). In the present paper, we report decay reactions resulting in C₃H₄⁺+H⁺, and C₃H₃⁺+H₂⁺. We observed fragment ions which are formed with high kinetic energy. Mass analyzed ion kinetic energy (MIKE) scan technique in the third field free region of the mass spectrometer was applied to identify the fragment ions and determine their kinetic energy release (KER). An average KER of 1.6±0.3 eV for the first reaction and 0.67±0.15 eV for the second reaction are reported.     

Magnetic Properties of Some Fullerene Compounds

Abstract

Magnetic properties of some fullerene intercalation compounds are detailed. A few examples are presented including properties of 1) Acceptors: MoF₆ From magnetic measurements the existence of positively charged C₆₀ (C₆₀ ⁺) can be inferred 2) Donnors: Yb_xC₆₀ and Eu₃C₆₀ compounds. Magnetic properties of (Yb_xC₆₀) are dominated by crystal field effects. A high field magnetic transition (17 T at 4 K) occurs in Eu₃C₆₀ associated with a large hysteresis of the magnetization and relaxation effects. This complex magnetic behaviour is attribued to Eu²⁺.     

Kinetic energy release and mean life time of metastable ions produced from C3H3N

Metastable ions C₃H_nN⁺ (n = 1, 2 or 3) and C₂H₂⁺ produced by electron impact on the neutral C₃H₃N (acrylonitrile) undergo metastable decay reactions resulting in the fragment ions C₃H_mN⁺ (m = 1, 2) or C₂H₂⁺. We have monitored these reactions in a double focusing mass spectrometer of reversed B-E geometry. To identify the fragment ions and determine their kinetic energy release distribution (KERD), the mass-analyzed ion kinetic energy (MIKE) scan technique was utilized. For the above mentioned ions we obtained average KER values ranging from 9 up to 23 meV. Moreover, the fractions of decaying ions were measured. We have found the existence of two states of the metastable C₂H₂⁺ ion, one with a short life time of about 0.27 μs and the other one which is most likely a completely stable ion state.     

Material dependence of electronic sputtering induced by MeV-energy heavy ions

Masses, yields and emission energies of secondary ions have been systematically measured for solid targets bombarded by Si ions over an energy range between 0.5 and 5 MeV for studying roles of the electronic collision in the formation process of secondary atomic ions. The targets used were insulators (Al₂O₃, SiO₂), semiconductors (Si, GaP, GaAs, GaSb and InSb) and a metal (Al). The obtained feature of the secondary ion emission depends characteristically on the target species. Singly and multiply charged positive ions are produced for Al, Si and P, respectively, from the Al, Al₂O₃, Si, SiO₂ and GaP targets. In the cases of GaAs, GaSb and InSb, however, the production rates of positive As and Sb ions are strongly depressed. The obtained emission energy distributions of atomic ions from the conductor and the semiconductors are very broad and have gradually decaying tails at the high-energy side in contradiction to those of singly charged atomic and cluster ions from the insulators. The most probable and mean energies of atomic ions are proportional to their electric charge irrespective of the target conductivity. Large cluster ions are produced from the SiO₂, Al₂O₃, GaSb and InSb targets. The yields of clusters from SiO₂, GaSb and InSb show power-law dependences on their sizes and the yield of clusters from Al₂O₃ decreases exponentially with increasing size. These results show that the clusters from SiO₂, GaSb and InSb are emitted directly from the surface. The clusters from Al₂O₃ may be indirectly produced by coagulation of molecules in a selvage region near the surface.     

Vacuum discharge as an effective source of multiply charged ions

The time-of-flight (TOF) spectra measured under high vacuum conditions revealed ion beams of a cathode material (Cu ⁿ⁺) with a maximum charge of up to +19 generated in the initial stage of a spark discharge development at a storage voltage of up to U ₀=2.5 kV. As the U ₀ value increases, the range of the multiply charged states of ions detected by the TOF technique increases, the average ion charge reaching +9.     

Noncovalent complexes of Ih−C80 fullerene with phthalocyanines

The noncovalent interactions of I_h?C₈₀ fullerene with free-base and 3d transition M(II) phthalocyanines (where M = Mn, Fe, Co, Ni, Cu, Zn) were studied at the PBE-D/DNP level of density functional theory. The optimized complex geometries, formation energies and electronic parameters were analyzed and compared to those reported previously for similar dyads with C₆₀. In the complexes with I_h?C₈₀, the central metal atom (as well as one H atom of H₂Pc) is always coordinated to a C_6:6:6 atom of C₈₀ cage, exhibiting only one general interaction pattern. The shortest M^…C_C80 and N^…C_C80 distances are notably longer than in dyads with C_60, and the distortion of Pc macrocycle is less significant. In none of C₈₀-based dyads the formation of new coordination bonds by metal atoms was observed. The bonding strength for Pc–C₈₀ dyads varies approximately to the same degree of 20 kcal/mol as for their Pc–C₆₀ analogues, however negative formation energies for Pc–C₈₀ dyads are on average by 5–6 kcal/mol lower than for their C₆₀ analogues. While in closed-shell Pc–fullerene systems HOMO is usually found totally on Pc molecule, in the case of C₈₀-based dyads only a minor HOMO fraction can be detected for some dyads on Pc, with the major one always distributed over fullerene cage. As a result, the calculated HOMO-LUMO gap energies turn to be very low, around 0.1 eV. Analysis of spin density plots revealed that H₂Pc+C₈₀, NiPc+C₈₀ and ZnPc+C₈₀ dyads behave as closed-shell systems, like similar noncovalent complexes of Pcs with C₆₀.     

Adsorption of amino acids by fullerenes and fullerene nanowhiskers

Abstract

We have investigated the adsorption of some amino acids and an oligopeptide by fullerene (C₆₀) and fullerene nanowhiskers (FNWs). C₆₀ and FNWs hardly adsorbed amino acids. Most of the amino acids used have a hydrophobic side chain. Ala and Val, with an alkyl chain, were not adsorbed by the C₆₀ or FNWs. Trp, Phe and Pro, with a cyclic structure, were not adsorbed by them either. The aromatic group of C₆₀ did not interact with the side chain. The carboxyl or amino group, with the frame structure of an amino acid, has a positive or negative charge in solution. It is likely that the C₆₀ and FNWs would not prefer the charged carboxyl or amino group. Tri-Ala was adsorbed slightly by the C₆₀ and FNWs. The carboxyl or amino group is not close to the center of the methyl group of Tri-Ala. One of the methyl groups in Tri-Ala would interact with the aromatic structure of the C₆₀ and FNWs. We compared our results with the theoretical interaction of 20 bio-amino acids with C₆₀. The theoretical simulations showed the bonding distance between C₆₀ and an amino acid and the dissociation energy. The dissociation energy was shown to increase in the order, Val < Phe < Pro < Asp < Ala < Trp < Tyr < Arg < Leu. However, the simulation was not consistent with our experimental results. The adsorption of albumin (a protein) by C₆₀ showed the effect on the side chains of Try and Trp. The structure of albumin was changed a little by C₆₀. In our study Try and Tyr were hardly adsorbed by C₆₀ and FNWs. These amino acids did not show a different adsorption behavior compared with other amino acids. The adsorptive behavior of mono-amino acids might be different from that of polypeptides.     

Computer Simulation of Molecular and Electron Structure of C60 Fullerene Complexes with Twelve Half‐Sandwich Groups MC5H5 (M = Fe,Ru, Os)

Abstract

The C₆₀(MCp)₁₂ complexes of I _h ‐C₆₀ fullerene in which each of 12 MCp groups (M = Fe, Ru, Os; Cp = C₅H₅) is coordinated to one of the five‐membered cycles of C₆₀ via formation of η⁵–π‐bond are considered. Geometry and electron structure of these complexes are simulated by using of the PBE–DFT approach. Stability of complexes considered is evaluated. It is shown that energies of the M–C₆₀ bonds are increased in the sequence C₆₀(FeCp)₁₂, C₆₀(RuCp)₁₂, C₆₀(OsCp)₁₂.     

He+ ION BOMBARDMENT OF C60 FULLERENE: AN FT-IR AND RAMAN STUDY

ABSTRACT

C₆₀ fullerene films have been bombarded with He⁺ ions at 30?keV at room temperature in vacuum. The structural changes undergone by C₆₀ have been followed by both FT-IR and Raman spectroscopy. Raman spectroscopy was the most useful tool for this scope. It has been clearly discovered that at low radiation dose C₆₀ forms oligomers but at higher radiation doses it is converted into an amorphous carbonaceous matter. The implications of these results on the possible survival of C₆₀ fullerene in the interstellar space have been discussed briefly in connection with the previous results on the effects of various types of electromagnetic radiation over C₆₀.     

Charge distribution of ions in low-current vacuum-arc plasma

The charge distribution of vacuum-arc ions has been investigated for the arc between copper electrodes at currents varied from 8 to 60 A (with the arc voltage increased from 21 to 24 V). Plasma contains copper ions with charge states from +1 to +4. The mean ion charge state increases from 2.15 to 2.24 (i.e., by only 4%) due to a small increase in the fractions of multiply charged ions (Cu²⁺ and Cu³⁺) in the total ion flux. This conclusion does not contradict that of the ecton model of vacuum-arc cathode spot, according to which the number of cells in the spot that are due to microexplosions on the cathode increases proportionally to the arc current.     

Surface Ionization Mass Spectrometric Studies of the Li/C60 System

Abstract

We have studied the Li/C₆₀ system by the surface ionization mass spectrometry. We have investigated a possibility of the Li@C₆₀ formation via the collisions between Li⁺and C₆₀ ^? ions in a plasma state, which was predicted by an ab initio molecular dynamics simulation. The LiC₆₀ complex was unambiguously observed. Our results do not allow direct determination of the LiC₆₀ structure but indicate that Li is inside the C₆₀ cage. We have determined the ionization potential of Li@C₆₀ complex (IP = 5.9 ± 0.1 eV), which agrees well with the calculated value of the IP of Li@C₆₀.