Ion-molecule branching ratios at high temperature: Vibrational energy promotes formation of new channels in the reaction of O₂⁺ with CH₄

A High-Temperature Flowing Afterglow (HTFA) instrument has been modified to allow measurements of ion-molecule branching ratios at temperatures above 700 K for the first time. The technique is capable of temperatures at least as high as 1400 K, and here we report the branching ratios for the reactions of O²⁺ with CH₄ and CD₄ measured at 1400 K (for the reaction of O₂⁺ with CH₄, the rate constant was also measured). Comparison to work using drift tube and guided ion beam techniques shows that heating the CH₄ vibrations produces new reaction product channels and enhances others. In particular, HCO⁺, CH₃O⁺, and H₂O⁺ products are seen for the first time, and the H₃O⁺ product is much more intense in comparison with experiments on vibrationally cold CH₄. The previously observed products CH₃⁺ and CH₄⁺ are comparable to those found with vibrationally cold CH₄. We conclude that vibrational excitation promotes those channels with require extensive bond rearrangement. Vibrational energy is found not only to promote the new channels but also to enhance the overall reaction rate.