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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new tandem time-of-flight mass spectrometer is described which is designed to study the mass-, velocity-, and angle-resolved scattering of cluster ions from solid surfaces. Clusters are produced in a supersonic jet laser desorption/vaporization source, ionized either directly in the formation step or by subsequent photoionization of neutrals, mass selected in a primary time-of-flight region, and decelerated to the impact energy (50–1000 eV) close to the target surface. Cluster–surface collisions take place in a field-free region in order to determine both velocity and angular distributions of the scattered clusters and fragments with an independently pulsed, rotatable secondary time-of-flight mass spectrometer. Several surface targets can be mounted in the UHV scattering chamber (10−10 Torr base pressure) on a five-axis manipulator which, together with the rotatable secondary TOF, allows for independent variation of incident and scattering angles. Target surfaces can be cleaned by direct current heating and sputtering with an argon-ion gun. Surface structure and composition are assessed by low-energy electron diffraction (LEED) and Auger spectroscopy with a four grid reverse view LEED/Auger system. Surface collision experiments of fullerenes (C+60, C+70, C+76, C+84, ...) and metallofullerenes (La@xaC+82) with highly oriented pyrolitic graphite (HOPG) surfaces are described as examples for the performance of the instrument. Effects of surface contamination in the scattering of fullerenes from HOPG are described to demonstrate the need for thorough cleaning procedures in order to obtain reproducible results. © 1995 American Institute of Physics.
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  • 2
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Carbon cluster ion impact induced surface damage (500, 1850, and 4000 eV) of highly oriented pyrolytic graphite (HOPG) was imaged on an atomic scale with scanning tunneling microscopy. Penetration depth was accessed by oxygen etching of ion impact induced defects. Depending on ion energy, etched holes up to 6 monolayers deep were observed. Surface morphologies of HOPG crystals can be tailored by the described method promising many applications for surface science studies and nanostructuring. © 1995 American Institute of Physics.
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  • 3
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction of C+60 and C+70 ion beams with a surface of highly oriented pyrolitic graphite was investigated by probing the ionization and fragmentation rates of scattered species within a time window of 20 μs following impact. Neutralization/reionization and fragmentation behavior was observed and followed by a pulsed deflection field applied to the surface at variable delays after the collision event. An almost complete collisional neutralization of the incident projectile was found. For an impact energy of 140–180 eV, a significant part of the scattered species was found to reionize by delayed electron emission within the experimental time window. The associated reionization and fragmentation kinetics were modeled with a system of differential equations assuming a simple unimolecular reaction diagram. Rate constants for delayed ionization and fragmentation were calculated as functions of internal energy and respective activation energies with the "finite heat bath'' model (Klots) and the Rice–Ramsperger–Kassel–Marcus expression, respectively. The calculated and measured (deflection field delay dependent) ion intensities were compared in a fit procedure. The best fit led to an activation energy for the fragmentation of C+60 (C+60→C+58+C2) of 6.6±0.5 eV. This translates to an activation energy of 7.1±0.5 eV for the fragmentation of neutral C60 (using the experimentally determined ionization potential of C58). For C+70 we obtained an identical (within error) activation energy for fragmentation (C+70→C+68+C2) of 6.6±0.5 eV. © 1996 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 3638-3650 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Fragmentation of various fullerenes was studied by surface impact on highly oriented pyrolytic graphite at collision energies Ecol of 150–1050 eV/molecule. The projectiles C+60, C+70, C+76, C+84, and C+94 were formed by laser desorption of chromatographically separated samples, while large carbon clusters C+94, C+110, C+164 were produced by laser-induced coalescence reactions. Except at the highest impact energies, the fragment distributions consist of even numbered C+n species with abundance maxima similar to those observed in fullerene synthesis. With increasing Ecol, we observe a size evolution in the fragment distributions characteristic of a sequential fragmentation process. Simulated fragment distributions based on statistical rate theory and a sequential C2 loss mechanism reproduce the experimental data well up to a maximum Ecol. They are used to determine the mean energy transfer during surface impact as a function of collision energy as well as its dependence on several experimental parameters such as the nature (cleanliness) of the target surface, the internal energy of the incident ion, and the incident fullerene size. Both internal and kinetic energy of the incident ion are found to contribute to the observed fragmentation although with different efficiencies. For the higher fullerenes we find a tendency towards increasing transfer efficiency of incident kinetic to internal energy with increasing projectile size. Finally, above a size-dependent impact energy threshold, a transition to a different high energy fragmentation process is indicated by changes in the fragment distributions. These go from exclusively even numbered fullerene fragments at low impact energy to smaller even and odd numbered C+n fragments at high Ecol. It is suggested that this change indicates the formation of high energy, nonfullerene isomers. © 1996 American Institute of Physics.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microwave and infrared measurements have been made for the van der Waals complexes N2–CH3CCH and CO–CH3CCH, and their isotopomers. The infrared spectra are complicated by the effects of a slightly hindered internal rotation of the propyne unit. Tunneling motions arising from the N2 or CO molecules are not apparent in the infrared spectra, but in the case of N2 are seen in the microwave spectra. Analysis of the spectral data shows that the dimers are near slipped parallel in structure. Their dipole moments are almost parallel to the dimer b axis, with center-of-mass separations of 3.756(1) A(ring) for CO–CH3CCH and 3.708(2) A(ring) for N2–CH3CCH. (The uncertainties shown in parentheses are two standard deviations.) A simple model based on electrostatic interactions between rigid monomers gives dimer structures in agreement with the experimental assignments, and in conjunction with an approximate model of dispersion energy allows estimation of barriers to internal rotation.
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Positively and negatively charged coalescence products of C60 and C70 were characterized following pulsed laser desorption from fullerene thin films. Coalescence occurs by gas-phase reactions. Positive ions are generated by thermoionization of hot neutrals, while negative ions likely form by electron attachment to neutral species in the outermost region of the desorption plume. Thermoionization rate and surface-induced dissociation (SID) measurements are consistent with fullerene-like structures for coalescence products detected as positive ions.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microwave and infrared measurements are reported for the van der Waals complex Ar–CH3CCH and its isotopomers. The structure is T shaped with equilibrium center-of-mass separation of 3.73 A(ring) and an angle of 82° between the molecule symmetry axis and the van der Waals bond. The infrared and microwave spectra are complex due to the effects of a slightly hindered internal rotor. Analysis of the spectral data shows that the dipole moment is almost parallel to the dimer b axis. A modified vibration/rotation Hamiltonian that includes an internal rotor potential is used to show that the barrier to internal rotation is near 10.8 cm−1.
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  • 9
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High resolution stimulated Raman spectroscopy (SRS) has been used to examine N2 and N2/He free jet expansions and also equilibrium samples of N2 from 15 to 110 K. The jet spectra show the formation of large liquid clusters which supercool and subsequently freeze to form crystalline β-N2 solid and, in He expansions, undergo a further transformation to a partially annealed α-N2 form. CW-SRS frequency and linewidth data obtained for equilibrium samples of the condensed phases of N2 yielded frequency–temperature relations used in deducing internal temperatures for the clusters produced in the expansion experiments. Analysis of the cooling curves indicates a mean cluster diameter of 35 nm and favors a prompt freezing process rather than a gradual conversion of liquid to solid in a single cluster on the microsecond time scale of the experiments. Supercooling limits of 34 to 44 K are deduced for the liquid, far below the triple point temperature of 63.2 K at which equilibrium samples freeze. Some evidence for surface versus bulk contributions to the spectra is seen in the asymmetric line shapes observed for liquid clusters in the condensation region. The results show that the high spectral and spatial resolution of nonlinear Raman methods such as SRS and CARS provide a unique probe of the condensation processes in free jet expansions.
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  • 10
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
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