Knowledge Bank

The hydroperoxyl radical (HO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$) is a key reactive intermediate in the low-temperature ($&lt;$1000K)andhighpressure($\geq$1atm)oxidationchemistryofhydrocarbonsandoxygenatedfuels.Chemicalkineticmodelshavebeendevelopedforanumberoffuelsinthelow-temperatureandhigh-pressureregime,butvalidationofthesemodelsreliesonquantitativemeasurementsofradicalintermediatessuchasHO${2}$.InsituopticalmeasurementsofHO${2}$ at high-pressure ($\geq$1\; atm)\; by\; absorption\; spectroscopy\; are\; complicated\; by\; spectral\; overlap\; from\; non-radical\; species.\; Faraday\; rotation\; spectroscopy\; (FRS)\; is\; a\; sensitive\; magneto-optical\; dispersion-based\; technique\; that\; can\; be\; used\; to\; selectively\; observe\; signals\; from\; radicals\; while\; strongly\; suppressing\; signals\; from\; non-radicals,\backslash textbf\{72\},\; 6602\; (1980).\}\; effectively\; eliminating\; the\; problem\; of\; spectral\; overlap\; at\; high-pressures.\; Recently\; FRS\; has\; been\; used\; in\; the\; first\; direct\; measurements\; of\; HO$_{2}$ at the exit of an atmospheric flow reactor during the oxidation of dimethyl ether (DME). An external-cavity quantum cascade laser was used to record FRS spectra of Q-branch transitions in the $\nu 2$ vibrational band at 7.14 $\mu$m. To describe the experimental signals a numerical spectral model was developed using line positions and intensities provided by the HITRAN 2008 spectral database. HO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ concentrations were determined through non-linear fitting of the spectral model to the experimental spectra. In this talk the non-linear fitting of the FRS spectra to the numerical model will be presented, and the measured temperature dependence on the concentration will be compared to results from chemical kinetic modeling of DME oxidation.