Combustion chemistry involves many exothermic reactions, which produce hot molecules. For these vibrationally or electronically excited molecules, emission-based time-resolved Fourier transform spectroscopy (TR-FTS) can be a fruitful way to study the reaction rate, product state distribution, and branching ratios of elementary combustion reactions. TR-FTS yields measurements of thermal rate coefficients as a function of temperature and pressure, and its inherently multiplexed nature makes it possible to simultaneously measure product branching ratios, internal energy distributions, energy transfer, and spectroscopy of radical intermediates. Together with total rate coefficients, this additional information provides further constraints upon and insights into the potential energy surfaces that control chemical reactivity.
While there has been great progress in gas phase emission-based TR-FTS, David Osborn is also developing absorption techniques because they are more general than emission-based methods and are not complicated by fluorescence lifetime effects or predissociation. Another thrust of this program is toward kinetic measurements of larger molecules. The development of absorption-based TR-FTS in the mid infrared “fingerprint” region will enable reactivity studies of larger hydrocarbons (C3—C6) found in practical fuels.
Time-resolved infrared emission of the HCCO + O2 reaction. The sharp fin of emission near time zero is from the reactant HCCO. Emission of the products CO and CO2 grows (due to chemical production of these molecules) and then decays (due to vibrational energy transfer) as a function of time. The much faster rate of vibrational cooling of CO2 vs CO is graphically demonstrated by the time-resolved blue-shifting emission leading to the band origins of the CO emission (2,143 cm–1) and CO2 emission (2,349 cm–1).