Journal of Electron Spectroscopy and Related Phenomena 126 (2002) 191–202 www.elsevier.com / locate / elspec Clustering at high temperatures: carbon formation in combustion a, b c * K. Siegmann , K. Sattler , H.C. Siegmann a Particle Technology, TU Delft, Julianalaan 136, NL-2628 BL Delft, Netherlands b Department of Physics and Astronomy, University of Hawaii at Manoa, 2505 Correa Road, Honolulu, HI 96822, USA c Stanford Synchrotron Radiation Laboratory ( SSRL), Stanford, CA 94309, USA Abstract The formation of carbonaceous particles in laminar, atmospheric pressure diffusion flames is investigated using laser ionization time-of-flight mass spectroscopy and on-line analysis of nascent-particle properties while the particles are still in gas suspension at atmospheric pressure. This latter analysis includes photoelectric yield spectroscopy of the particle surface. Combustion gases from the, e.g. argon-diluted methane (CH ) flame are extracted and diluted with an inert gas prior to 4 analysis. The molecules are ionized by two-photon photoemission inside a time-of-flight mass-spectrometer which makes it possible to observe polycyclic aromatic hydrocarbons (PAHs) at molecular masses up to |600 amu. The mobility diameter of the carbonaceous particles was determined in the range from 2 to 20 nm. Formation and destruction of particles and their concomitant density and surface transformations are studied together with the PAH molecules as they depend on the height in the flame. It is found that particles are formed before large PAHs appear. As opposed to previous models where the flat PAHs were generated in homogeneous gas phase reactions, we conclude that PAHs are synthesized on the particles using their surface as templates and evaporate into the gas-phase when synthesis is completed. We present a comprehensive, new mechanism for particle formation in diffusion flames.  2002 Elsevier Science B.V. All rights reserved. Keywords: Soot; Flame; Polycyclic aromatic hydrocarbons; Photoionization; Aerosol 1. Introduction combustion. The latter process is usually referred to as ‘soot formation’. Soot, however, is a side product The carbon atom is unique among all other in the process of energy production, and is unwanted elements because it forms thermodynamically and because it exerts a large warming influence on global kinetically extremely stable bonds to hydrogen- or climate [1], and soot particles in the atmosphere are other carbon atoms. This is the reason for the found to be a serious hazard to human health (see, formation of solid carbon particles even in a high- for example cancer [2], mortality [3], and allergies temperature environment as it is found in incomplete [4]). Because our civilization still relies on the combus- tion of fossil fuels for power generation, the forma- tion of soot has a tremendous impact on mankind. Yet, soot formation is still inadequately understood. The current model of soot formation [5] is summa- *Corresponding author. E-mail address: k.siegmann@tnw.tudelft.nl (K. Siegmann). rized below. 0368-2048 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048(02)00152-4