Fragmentation of fullerenes in collisions with atomic and molecular targets

New insight into power-law behavior of fragment size distributions in the C₆₀ multifragmentation regime

Previous experimental work has shown that a phase transition in C60 multifragmentation induced by nanosecond laser occurs at almost constant temperature covering a wide range of laser fluency. Here the relative yields of ionic fragments (IFs) C(n)(+) (n = 1-20) resulting from the multifragmentation are measured within the phase transition region. By excluding two small IFs and magic IFs due to their abnormal behavior, the data for residual IFs are used to estimate the size distributions of primary intermediate-mass IFs in the multifragmentation regime. The distributions are found to obey power laws n(-τ). Furthermore, the exponent τ values have sensitive dependence on lower laser fluency and converge to a constant of about 2.4 ± 0.2 for larger fluencies. These observations are in good agreement with an explanation based on the Fisher droplet model, offering the tantalizing possibility of a liquid-to-gas phase transition in C60 systems.

Collision Energetics in a Tandem Time-of-Flight (TOF/TOF) Mass Spectrometer with a Curved-Field Reflectron

Collisions of fullerene ions (C(60) (+)) with helium and neon were carried out over a range of laboratory energies (3-20 keV) on a unique tandem time-of-flight (TOF/TOF) mass spectrometer equipped with a curved-field reflectron (CFR). The CFR enables focusing of product ions over a wide kinetic energy range. Thus, ions extracted from a laser desorption/ionization (LDI) source are not decelerated prior to collision, and collision energies in the laboratory frame are determined by the source extraction voltages. Comparison of product ion mass spectra obtained following collisions with inert gases show a time (and apparent mass) shift for product ions relative to those observed in spectra obtained by metastable dissociation (unimolecular decay), consistent with impulse collision models, in which interactions of helium with fullerene in the high energy range are primarily with a single carbon atom. In addition, within a narrow range of kinetic energies an additional peak corresponding to the capture of helium is observed for fragment ions C(50) (+), C(52) (+), C(54) (+), C(56) (+) and C(58) (+).

Adsorption of polar molecules on krypton clusters

The formation process of binary clusters has been studied using synchrotron based core level photoelectron spectroscopy. Free neutral krypton clusters have been produced by adiabatic expansion and doped with chloromethane molecules using the pickup technique. The comparison between the integrated intensities, linewidths, and level shifts of the cluster features of pure krypton and of chloromethane-krypton clusters has been used to obtain information about the cluster geometry. We have shown that most of the chloromethane molecules remain on the surface of the clusters.

Infrared multiphoton ionization of superhot C60: experiment and model calculations

We address, both experimentally and theoretically, the issue of infrared (IR) resonance enhanced multiphoton ionization (IR-REMPI) and thermally induced redshifts of IR absorption lines in a very large and highly vibrationally excited molecular system. Isolated superhot C60 molecules with well defined and variable average vibrational energy in the range of 9-19 eV, effusing out of a constant flux thermal source, are excited and ionized after the absorption of multiple (500-800) infrared photons in the 450-1800 cm(-1) spectral energy range. Recording the mass-selected ion signal as a function of IR wavelength gives well resolved IR-REMPI spectra, with zero off-resonance background signal. An enhancement of the ion signal of about a factor of 10 is observed when the temperature is increased from 1200 to 1800 K under otherwise identical conditions. A pronounced temperature dependent redshift of some of the IR absorption lines is observed. The observations are found to be in good agreement with a model which is based on the sequential absorption of single photons, always followed by instantaneous vibrational energy redistribution. The mass spectra (C60(+) fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C32(+) is observed following the absorption of 1350-1400 cm(-1) as well as 1500-1530 cm(-1) photons while negligible fragmentation is observed when exciting around 520 cm(-1).

Direct measurement of internal energy of fragmented C60

A new experimental approach has been investigated to measure directly the internal energy of fragmented C60. Doubly charged C60{2+*} prepared in collisions H++C60-->H- +C60{2+*} decay by evaporation of C2 units. We have measured the internal energy distribution of the transient C60{2+*} for each decay channel by analyzing the kinetic energy loss of the scattered anion H-. This method offers an experimental opportunity for studying the fragmentation dynamics of more complex systems such as larger clusters and biomolecules under internal energy controlled conditions.

Ionization and fragmentation of solid C60 by femtosecond laser ablation

Ionization and fragmentation of solid C(60) dispersed on a silicon plate are investigated by femtosecond laser ablation. Bimodal mass distribution with large fragment ions C(60-2n) (+) (0< or =n< or =11) and small fragment ions C(n) (+) (13< or =n< or =28), formation of dimer ion (C(60))(2) (+), and delayed ionization of C(60) have been observed as reported in gas phase experiments with nanosecond laser excitation. Metastable dissociation of small fragment ions C(n) (+) has been observed for the first time, which suggests different structures of fragment ions compared with those of well-studied carbon cluster ions. From these observations, strong coupling of laser energy to electronic degrees of freedom of solid C(60) has been revealed for femtosecond laser ablation as compared with excitation in the gas phase.

Sputtering of C60 Fullerenes Physisorbed on Ar, Xe, H2O, O2, and C8F18 Matrix Films Studied with Time-of-Flight Secondary Ion Mass Spectrometry

The ionization and fragmentation of C(60) fullerenes were investigated using matrix films covered with C(60) molecules and bombarded with 1.5-KeV He(+) ions. C(+), C(60)(+), and C(60)(++) ions were sputtered from the C(60) molecules that were physisorbed on Ar and Xe matrix films, whereas the sputtering of C(60) on the O(2) and C(8)F(18) matrix films induced an additional emission of ion adducts, such as (OC(60))(+) and (FC(60))(+), as well as the fragment ions, C(60-2n)(+) (n = 1-10). Very few ions were sputtered from the C(60) molecules that were adsorbed on the H(2)O matrix film and the Ni(111) substrate. The ions are thought to be created at the surface when C (C(60)) collides with the Ar, Xe, O, and F species via the electron-promotion mechanism, and the formation of quasi-molecules is manifested from the emission of the ion adducts. The fragmentation occurs during the interaction with the reactive species at the surface, and the delayed ionization/fragmentation of the internally excited C(60) molecules in the gas phase has negligible contribution in the present experiment. The matrix effect arises from the suppressed neutralization of the C(60)(+) ion because of the localization of a valence hole. The C(60)(+) ion undergoes neutralization on the H(2)O film because the hydrogen bond has some covalent character.

Event-by-event analysis of collision-induced cluster-ion fragmentation: sequential monomer evaporation versus fission reactions

The most abundant decay channels have been studied quantitatively for high-energy (60 keV/amu) cluster ions H (3) (+)(H (2))(m = 1-14) colliding with He atoms employing a recently developed multicoincidence technique for the simultaneous detection of the correlated fragments on an event-by-event basis. This allows us to identify decay reactions and their underlying decay mechanisms responsible for the occurrence of the U-shaped fragmentation pattern.

High-energy collision-induced dissociation of small polycyclic aromatic hydrocarbons

High-energy collision-induced dissociation (CID) experiments on polycyclic aromatic hydrocarbons (PAHs) having 2-6 rings, naphthalene, anthracene, phenanthrene, fluoranthene, pyrene and coronene, were performed, and the relative abundances of their fragment ions were investigated as a function of collision energy. The results revealed that the PAHs except naphthalene showed a bimodal-type distribution of positive fragmentation ions, which is closely similar to the fragment-ion distribution reported for the CID of three-dimensional fullerene, C(60)(+) and C(70)(+). The three-ring isomers of anthracene and phenanthrene and the four-ring isomers of fluoranthene and pyrene can be distinguishable in their spectra under an electron ionization energy of 70 eV, but the high-energy CID spectra of the three- and four-ring isomers were almost identical. The fragmentation corresponding to fragment ions in the low-mass region of the bimodal CID spectra could be interpreted by the simple statistical model that fragment ions are formed by random evaporation from the molecular ions after a considerable structural rearrangement, 'phase transition', occurring at some high-energy state.