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 un der discussion. Reasonable candidates are structures containing silico n 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 du st. Light scattering, fluorescence induced by UV laser and light absor ption have been employed to analyse phenomenological aspects of format ion, destruction and chemical transformation of carbonaceous structure s formed in rich premixed laminar methane-oxygen and ethylene-oxygen f lames at atmospheric pressure with different C/O ratios at and above t he soot threshold limit. Light scattering measurements also show that in non-sooting conditions, i.e. in the non-sooting zone of sooting fla mes or in flames below the soot threshold limit, high molecular mass s tructures are formed. Absorption spectra measured in non-sooting condi tions show a continuous decay from 200 to 300 nm, while in fully sooti ng conditions an additional peak at around 235 nm appears. Further inf ormation is obtained by laser-induced fluorescence investigations. Her e, a broad-band peak with a maximum at around 320 nm is always measura ble. Approaching the soot nucleation zone a further broad-band fluores cence emission in the visible, in addition to that in the UV, is obser ved. All the experimental results produce evidence that the early form ation of high molecular mass carbonaceous molecules containing aromati c functionalities with not more than two rings occurs. Their role as s oot precursors seems to be connected to their change into higher polyc ondensed aromatic structures. Our results give a hint that these soot precursors and the interstellar dust are members of the same family of carbonaceous structures.