| Popis: |
A quadrupole mass spectrometer was used to measure individual and total ion concentrations in premixed atmospheric-pressure flames of acetylene, oxygen and argon, to which small quantities of sodium, strontium, or manganese had been added. Large increases in total flame ionization were noted. Although chemi-ionization mechanisms can account for such an increase with strontium and manganese addition, some other mechanism (e.g., collisional ionization by “hot” electrons) is needed to explain the rapid formation of sodium ions. All three metals accelerated the rate of disappearance of natural flame ions of mass < 170 amu. This was because of charge exchange reactions between such flame ions and neutral metallic species, as well as enhanced recombination of these flame ions with free electrons. Sodium and strontium also reduced the average size of charged soot particles, whereas manganese had no observable effect. The observations strongly suggest that small chemi-ions, such as C3H3+, play only a very minor role in soot nucleation and that it is larger primary chemi-ions with masses of a few hundred amu, which are the key nuclei in an ionic mechanism of soot formation. These big chemi-ions are typically formed by reactions like C16H10 + C6H → C22H11+ + e−; they are removed by charge exchange with Na or Sr species, but not ones of Mn with their relatively high ionization potentials. This explains how Na or Sr addition (ionization potentials relatively low) can reduce the rates of soot nucleation and consequent particle growth. |
