THE OPTICAL CHARACTERIZATION OF HIGH-MOLECULAR-MASS CARBONACEOUS STRUCTURES PRODUCED IN PREMIXED LAMINAR FLAMES ACROSS THE SOOT THRESHOLD LIMIT

The consistence of the dark material in interstellar space is still under discussion. Reasonable candidates are structures containing silicon or carbon. In that light it is interesting to know the physical and chemical properties of carbonaceous structures produced in flames and to compare them with the spectroscopical properties of interstellar dust. Light scattering, fluorescence induced by UV laser and light absorption have been employed to analyse phenomenological aspects of formation, destruction and chemical transformation of carbonaceous structures formed in rich premixed laminar methane-oxygen and ethylene-oxygen flames at atmospheric pressure with different C/O ratios at and above the soot threshold limit. Light scattering measurements also show that in non-sooting conditions, i.e. in the non-sooting zone of sooting flames or in flames below the soot threshold limit, high molecular mass structures are formed. Absorption spectra measured in non-sooting conditions show a continuous decay from 200 to 300 nm, while in fully sooting conditions an additional peak at around 235 nm appears. Further information is obtained by laser-induced fluorescence investigations. Here, a broad-band peak with a maximum at around 320 nm is always measurable. Approaching the soot nucleation zone a further broad-band fluorescence emission in the visible, in addition to that in the UV, is observed. All the experimental results produce evidence that the early formation of high molecular mass carbonaceous molecules containing aromatic functionalities with not more than two rings occurs. Their role as soot precursors seems to be connected to their change into higher polycondensed aromatic structures. Our results give a hint that these soot precursors and the interstellar dust are members of the same family of carbonaceous structures.