In the last two decades, high temperature (T> 1000 K) rate constants for several key combustion reactions involving atomic species (H, O, I, Cl ...etc.) have been measured in this laboratory using reflected shock heating coupled with the atomic resonance absorption spectrometric (ARAS) detection technique. The use of ARAS with its high sensitivity almost always has eliminated the need for chemical kinetics modeling. Hence, these earlier studies were considered to be direct. Similar high temperature kinetics studies involving important combustion radicals (OH, CH3 and CF2) were also performed using optical absorption detection techniques. However, low absorption coefficients of radicals, compared to atomic species result in a substantial decrease in sensitivity and hence warrant the need for detailed chemical kinetics modeling.

Recently, we have used a novel multi-pass absorption technique for sensitive time-resolved monitoring of OH-radicals at high temperatures. The increase in sensitivity is achieved by multiple reflection of the monitoring light between two concave reflectors mounted on either side of the shock tube apparatus. Applying this method has resulted in gradual improvement of the number of passes from 12 to 80. The present optical configuration gives a S/N ratio of ~1 at ~0.5-1.0 x 1012 radicals cm-3. Hence, kinetics experiments could be performed at sufficiently low [OH]0 = ~ 4-20 ppm thereby minimizing secondary reactions. We have used this technique in measuring rate constants for several important unimolecular dissociation processes (H2O + Kr, CH3OH + Kr and CF3 + Kr), hydrocarbon oxidation (CH4+O2, CH3 + O2 and CH2O+O2) reactions and bimolecular OH depletion (OH + C2H2, C2H4, NO2, CF3H..etc) reactions some of which will be discussed. In most cases, we have used a reaction mechanism scheme coupled with a sensitivity analysis procedure to obtain profile fits that often agreed with the experiment to within <±10 %. Most of these results have either been compared to earlier determinations or with recent high-level theoretical calculations to extend the existing kinetics database over a wider temperature range.