A global technique for analysing multiple decay curves. Application to the CH3+ O2 system

Abstract

Methyl radical decay profiles, in the presence of O₂, have been analysed by a global technique to obtain reliable estimates of the rate constants for the two contributing reactions CH₃+O₂→ CH₃O₂(1), CH₃+CH₂O₂→ 2CH₃O. (3)Single decay curves do not contain sufficient information for the independent determination of k₁ and k₃, but the simultaneous analysis of several decay curves, obtained for a range of [CH₃]_{t= 0}/[O₂] concentrations so that the relative contributions from reactions (1) and (3) vary, provides well determined estimates. A non-linear least-squares fitting technique was employed, incorporating the Marquardt algorithm, in which the fitting profiles were calculated either analytically (via and approximate solution, valid for [CH₃]_t= 0/[O₂]⩽ 10^–3) or by numerical integration. The technique confirms the validity of previously reported estimates of k₁, but provides much more precise estimates of k₃, and gives k₃= 8.4 × 10^–12exp(710K/T) cm³ molecule^–1 s^–1 over the range 298–530K. The data also enable the absorption cross-section for the methylperoxy radical to be determined, over the same temperature range. A temperature–independent value of 3.3 × 10^–18 cm² is obtained at 216.36 nm, for a 0.6 nm bandpass.