Shock‐Tube Study of the Acetylene–Oxygen Reaction. III. Absolute Rate of Chemi‐Ionization during the Induction Period

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.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 ...