Multiphoton Ionization Mass Spectroscopy of Fullerenes in Methane Diffusion Flames
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H. HEPP*, K. SIEGMANN,* AND K. SAYI'LER** *Laboratory for Solid State Physics, Swiss Federal Institute of Technology (ETH), CH-8093 Zuerich, Switzerland *Department of Physics and Astronomy, University of Hawaii, Honolulu, HI
ABSTRACT Multiphoton ionization time-of-flight mass spectrosocopy is used as an on-line technique to investigate polycyclic aromatic hydrocarbons (PAH) and fullerenes in atmospheric pressure flames. We have recorded height profiles of neutral PAH and fullerenes in methane/argon diffusion flames. Fullerenes ranging from C32 to at least C150 are found in a region where PAH concentration starts to decrease. Uniformity of the profiles indicates that the fullerenes do not grow from smaller to larger ones. The influence of the amount of argon additive is discussed.
INTRODUCTION Nucleation and growth of carbon clusters at high temperature is a unique process which is usually not observed with other cluster materials. A famous example for this process is the production of macroscopic quantities of fullerenes by graphite vaporization1 . Soot formation in flames is an even more familiar phenomenon of high temperature carbon growth. Although mostly carbon-hydrogen compounds are found in the complex machinery of flames, the possibility of fullerene formation in flames has been suggested 2 already shortly after the discovery of this modification of carbon. The first fingerprints of fullerenes in flames were observed by mass spectrometry of their ions probed from low-pressure premixed benzene/oxygen and acetylene/oxygen flames 3 ,4 . Later, also the corresponding neutral species were measured in these flames by multiphoton ionization mass spectrometry 5 . More evidence of the presence of fullerenes in this type of flame has been provided by Howard and coworkers using solvent extraction and spectroscopic techniques 6 . They have further shown that the amount of C6 0 and C70 formed in the flame is substantial and can be varied over a wide range by changing the burning conditions 7 . Hence, flame based fullerene synthesis may become interesting for production. Recently, mass spectra of fullerene ions have been obtained also from low-pressure premixed naphthalene/oxygen/argon flames 8 . To our knowledge, fullerenes had not yet been detected in diffusion flames. It had been conjectured that diffusion flames give higher yields of fullerenes then premixed flames 7 . Malhotra and Ross 9 studied soot sampled from an open butane flame using field ionization mass spectrometry but found no peaks corresponding to fullerenes. We have investigated a methane/argon diffusion flame at atmospheric pressure by on-line multiphoton ionization time-of-flight mass spectroscopy. Neutral molecular flame species are extracted using a narrow quartz glass capillary. In this way, we have measured high-resolution profiles of the species concentrations across the flame. Low laser intensity at a wavelength of 517 Mat. Res. Soc. Symp. Proc. Vol. 359 0 1995 Materials Research Society
250 nm was suitable for ionization of polycyclic aroma
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