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Isoprene is an unsaturated hydrocarbon and the most abundant non-methane hydrocarbon in Earth’s atmosphere. Atmospheric oxidation of isoprene via reaction with ozone leads to formation of carbonyl oxide intermediates, known as Criegee intermediates. Methacrolein oxide (MACR-oxide, (CH2=C(CH3))CHOO), is a four carbon unsaturated Criegee intermediate produced in isoprene ozonolysis. The aim of this study is to obtain the UV-Visible absorption spectrum of MACR-oxide on a strong π*¬π electronic transition associated with the conjugated carbonyl oxide and vinyl groups. MACR-oxide is synthesized in the laboratory by photolyzing a diiodo alkene precursor (1,3-diiodo-2- methylprop-1-ene) followed by reaction with oxygen that transiently forms a iodoalkene peroxy radical. Subsequent loss an iodine atom yields MACR-oxide in four conformational forms. MACR-oxide is produced in a quartz capillary reactor tube and cooled in a pulsed supersonic jet expansion. MACR-oxide is detected by photoionization using 118 nm VUV radiation on the m/z = 86 mass channel of a time of flight mass spectrometer. UV-Visible radiation is absorbed and promotes MACR-oxide from its ground state to an excited 11ππ* electronic state. This results in a ground state depletion that is detected as a reduction in the 118 nm photoionization signal. The UV-Visible spectrum of MACR-oxide is observed from 315 to 500 nm and peaks at 380 nm. Weak oscillatory structure is found in the long wavelength region of the spectrum, which can be attributed to vibrational resonances. Complementary Velocity Map Imaging experiments show that MACR-oxide dissociates rapidly upon UV-Visible excitation. This assures that the UV-Visible spectrum of MACR-oxide obtained by the depletion method is equivalent to a direct absorption measurement. The UV-visible spectrum of MACR-oxide is expected to have broad applicability as a sensitive probe of its unimolecular decay and bimolecular reactions with trace species relevant in the atmosphere.
Criegee Intermediates, Methacrolein Oxide, Electronic Spectroscopy, Photochemistry, Time of Flight Mass Spectrometry, Isoprene Ozonolysis, Gas Phase Spectroscopy
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Date Posted: 31 May 2019