We present application of chirped-pulse microwave spectroscopy in uniform supersonic flows (CPUF) to study product branching and kinetics in photodissociation and bimolecular reactions. This approach combines broadband rotational spectroscopy developed by Pate and coworkers, with low temperature supersonic flows as pioneered by Rowe and others. The combination offers unique advantages and opportunities for astrochemical problems. We will describe isomer-specific detection and product branching fractions in the UV photodissociation of the propargyl radical to C3H2 products propadienylidene (l-C3H2), triplet propargylene (3HCCCH) and the most stable, cyclopropenylidene (c-C3H2). At the same time, the results afford insight into isomerization of l-C3H2, and 3HCCCH to c-C3H2 induced by reaction with H atoms, a process that certainly occurs in interstellar environments and alters the relative abundances of these radicals. In these studies we employ a unique pulsed quasi-uniform flow that is optimized for microwave detection. Propargyl radicals are produced in the 193 nm photodissociation of 1,2-butadiene or other precursors. Absorption of a second photon leads to H atom elimination giving the three possible C3H2 isomers. The singlet products and their appearance kinetics in the flow are directly determined by rotational spectroscopy, but due to the negligible dipole moment of HCCCH, it is not directly monitored. However, we observe that both l-C3H2 and HCCCH undergo efficient H-atom catalyzed isomerization in the 20K flow to form c-C3H2. We exploit the time-dependent kinetics of this isomerization to infer the branching to the undetected HCCCH as well. We obtain the overall branching among H loss channels to be 2.9% (+1.1/-0.5) l-C3H2 + H, 16.8% (+3.2/-1.3) c-C3H2 + H, and 80.2% (+1.8/-4.2) 3HCCCH + H. Our findings are qualitatively consistent with earlier RRKM calculations in that the major channel in the photodissociation of the propargyl radical at 193 nm is to HCCCH + H; however, a greater contribution to the energetically most favorable isomer, c-C3H2 + H is observed in this work. We do not detect the predicted HCCC+H2 channel, but place an upper bound on its yield of 1%. Future applications of the method to other problems of astrochemical interest will also be discussed.

Chirped-pulse microwave spectroscopy in uniform supersonic flows: Isomer-specific branching in photodissociation of propargyl radical

256th National Meeting (2018)

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