Flames / Fullerenes 1 Ionization / Mass SpectrometryPositive and negative fullerene ions from C32 to over Cm have been analysed in low-pressure premixed flames of acetylene and benzene with oxygen. Mass spectra were taken through a molecular beam sampling system by a linear time-of-flight mass spectrometer (TOF-MS) and a reflectron TOF-MS. Fullerenes are mainly formed during the beginning of soot formation, and less in the oxidation zone or under nonsooting conditions in benzene flames. When soot is present, the whole spectrum of fullerene ions is formed, whereas in the oxidation zone mainly smaller fullerenes, preferentially Cm, C7, , and Cm occur. -Polycyclic aromatic hydrocarbons (PAH) are the precursors of fullerenes, both in the Gresence of soot and under homogeneous conditions. However, hydrogenated fractions of carbon cages as intermediates could not be detected. This is attributed to fast unimolecular reactions and to formatian reactions of fullerenes in young soot particles. On account of correlations with concentration profiles of large PAH ions, on the one hand, and with the soot particle mass distributions, on the other, possible mechanisms of fullerene formation are discussed. Fullerenes do not grow further, as do the soot particles, but they are oxidized at high temperature. Thermal decomposition was not observed. From comparison of fullerene formation with the two fuels and under different burning conditions it is concluded that negative fullerene ions are formed by electron attachment to neutral fullerenes and that for positive fullerene ions there is hardly any ionisation equilibrium.
ExperimentalThe pre-mixed flat flames of acetylene/oxygen and benzene/oxygen burned on a vertically movable, cooled sintered-disk burner of 75 mm diameter at a standard pressure of 2.66 kPa (20 Torr) as used previously [4]. Acetylene and oxygen were admitted through capillary flow meters. For the benzene flames, the fuel was gasified by saturating a stream of pure oxygen with benzene in a three-stage saturator-condenser [5, 61. Flame ions of both signs were extracted from the flame by a molecular beam sampling system and analysed by a time-of-flight mass spectrometer (linear TOF-MS) [4, 71. Briefly: Samples were drawn through a Pt-coated quartz nozzle probe (0.70 mm diameter)which was kept electrically at ground potential. A molecular beam containing the ions was formed by a skimmer and entered a second chamber. A 300 V pulse extracted the ions from the molecular beam. They were accelerated to an energy of 2.6 keV. After 53 mm of flight they hit a CsI-covered Al-plate which served as a conversion dynode forming secondary electrons amplified by a channeltron detector. The signal was fed into a pre-amplifier through a ceramic Ber. Bunsenges. Phys. Chem. 96 (1992) No. 7 0 VCH Verlagsgesellschaft mbH. W-6940 Weinheim, 1992 0005-9021/92/0707-0841 8 3.50+ ,2510 Th. Baum et al.: Fullerene Ions and Their Relation to PAH and Soot etc.capacitor. A box-car averager was used for amplification and scanning of the mass spectrum.R...
Computer simulations of the dissociation dynamics of argon and xenon neutral and singly charged clusters with 5 -20 atoms were performed. It is shown that the stability of clusters with 'magic numbers' of atoms (« = 19 for argon, and n = 13,19 for xenon) as found in TOF mass spectra, is more enhanced by ionization than that of their neighbours, indicating the importance of the ionization for the cluster size distribution found in experiments.
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