Thermal decomposition of CH ₃ I revisited: Consistent calibration of I-atom concentrations behind shock waves with dual I-/H-ARAS

Iodinated hydrocarbons are often used as precursors for hydrocarbon radicals in shock-tube experiments. The radicals are produced by C─I bond fission reaction, and their formation can be followed through time-resolved monitoring of the complementary I-atom concentrations, for example, by I-atom resonance absorption spectroscopy (I-ARAS). This very sensitive technique requires, however, an independent calibration. As a very clean source of I atoms, CH ₃ I is particularly well suited as calibration system for I-ARAS presumed the yield of I atoms and the rate coefficient of I-atom formation from CH ₃ I are known with sufficient accuracy. But if the formation of I atoms from CH ₃ I by I-ARAS is to be characterized, an independent calibration system is required. In this study, we propose a cross-calibration approach for I-ARAS based on the simultaneous time-resolved monitoring of I and H atoms by ARAS in C ₂ H ₅ I pyrolysis experiments. For this reaction system, it can be shown that at sufficiently short reaction times very similar amounts of I and H atoms are formed (difference <1%). As calibration of H-ARAS, with mixtures of N ₂ O and H ₂ , is a well-established technique, we calibrated I-atom absorption–time profiles with respect to simultaneously recorded H-atom concentration–time profiles. Using this approach, we investigated the thermal decomposition of CH ₃ I in the temperature range 950–2050 K behind reflected shock waves at two different nominal pressures (p ∼ 0.4 and 1.6 bar, bath gas: Ar). From the obtained absolute I-atom concentration–time profiles at temperatures T < 1250 K, we inferred a second-order rate coefficient k(T) = (1.7 ± 0.7) × 10 ¹⁵ exp(–20020 K/T) cm ³ mol ^–1 s ^–1 for the reaction CH ₃ I + Ar → CH ₃ + I + Ar. A small mechanism to describe the pyrolysis of CH ₃ I under shock-tube conditions is presented and discussed.