Infrared emission accompanying the gas phase recombination of alkyl radicals

Abstract

Infrared emission from nascent products of the methyl radical recombination reaction has been detected using time-resolved Fourier transform emission spectroscopy. Methyl radicals were generated by 193 nm photolysis of acetone, and at low pressures emission from nascent photofragments (CH₃ and CO) is found to dominate. At higher pressures an additional emission band is seen in the C–H stretching region, the time dependence of which shows a rapid rise and a slow decay, the latter being dependent on the laser intensity and accurately described by a second-order fit. The emission is assigned to vibrationally excited ethane formed by recombination of methyl radicals, with the decay identified as the recombination rate and the rapid rise as quenching of C₂H₆ to levels which have no excitation in the emitting mode (the ν₅ C–H stretch). The recombination rate constants were determined from fits to the decays together with measurements of the laser fluence, and agree well with previous determinations. The wavelength dependence of the C₂H₆ emission spectrum is invariant with time, and is always red-shifted with respect to a room-temperature C₂H₆ absorption spectrum. This can be rationalised in a model which recognises that formation of vibrationally excited C₂H₆ is considerably slower than quenching, and from which it is found that the average level of excitation per excited ethane molecule reaches a steady-state value after a short induction period. Similar spectra and kinetics were observed with other methyl radical sources, and emission from the products of the ethyl radical recombination/disproportionation reaction was also observed when diethyl ketone was photolysed at 193 nm.