Abstract
The absolute specific rate of chemi-ionization during the induction period of the C2H2–O2 reaction has been measured using a new end-on shock-tube technique. Ion production was monitored using three gas mixtures with [O2]/[C2H2] ratios of 1.5, 3.0, and 6.0 over the temperature range 1409–2467°K. The data from each experiment was used to calculate differential ion yield at 10% reaction. The differential ion yields (ion pairs formed/C2H2 molecule consumed) were found to be independent of density and mixture composition within experimental error. However, the ion yields were found to depend strongly on temperature varying from near 4 × 10−9 at 1400°K to 2 × 10−6 at 2500°K. The exponential growth constants for electron production during the induction period were found to be a factor of 2 greater than those for CO production in each experiment above 1700°K. This relationship verifies the chemi-ionization process to be second order in reaction intermediates which is consistent with the reaction CH+O→CHO++e and rules out the reaction CH*+C2H2→C3H3++e. A theoretical expression for the differential ion yield is derived using an elementary model mechanism for the induction period of a chain-branching reaction. The data obtained in this study are compared with the predictions of the model mechanism.
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