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\title{PEPICO ANALYSIS OF C$_6$H$_6$ ISOMERS PRODUCED THROUGH THE RECOMBINATION OF PROPARGYL (C$_3$H$_3$) RADICALS}

\author{\underline{\FirstName H.R. \LastName HRODMARSSON}}
\address{Laboratoire Interuniversitaire des Syst\`emes Atmosph\'eriques, CNRS UMR 7583, Universit\'e Paris-Est Cr\'eteil et Universit\'e
Paris Cit\'e, IPSL, 61 av. G\'en\'eral de Gaulle, 94010 Cr\'eteil, France}

\author{\FirstName G.A. \LastName GARCIA, \FirstName L. \LastName NAHON, \FirstName L. \LastName BOUREHIL}
\address{Synchrotron SOLEIL, LOrme des Merisiers, St Aubin, BP 48, Gif sur Yvette, France}

\author{\FirstName B. \LastName GANS, \FirstName S. \LastName BOYE-PERONNE}
\address{Institut des Sciences Mol\'eculaires dOrsay, CNRS, Universit\'e Paris-Saclay, 91405 Orsay, France}

\author{\FirstName J.-C. \LastName GUILLEMIN}
\address{Institut des Sciences Chimiques de Rennes, CNRS UMR 6226, Universit\'e Rennes 1, 263 Avenue G\'en\'eral Leclerc, 35700 Rennes, France}

 \author{\FirstName J.-C. \LastName LOISON}
\address{Institut des Sciences Mol\'eculaires, UMR 5255 CNRS---Universit\'e de Bordeaux, F---33405 Talence Cedex, France}

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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
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\maketitle
The growth of polycyclic aromatic hydrocarbons (PAHs) is initiated by the formation of the first closed-ring species and is the quintessential step in incipient particle formation in aliphatic fuels. Among the most prominent formation pathways is the recombination of two propargyl (C$_3$H$_3$) radicals. Experimental validation of the recombination of propargyl radicals and a quantification of the benzene exit channel is challenging due to the complexity of the propargyl self-reaction and the number of possible intermediates and final products. A universal real-time method of detection is necessary to account for multiple C$_6$H$_6$ species simultaneously and in real-time. Mass spectrometry coupled to tunable vacuum ultraviolet (VUV) radiation from synchrotron sources has been used successfully in the past for species determination in complex media. Recently, photoionization efficiency curves were used to tentatively infer the formation of benzene molecules from the propargyl self-reaction along with a more clear detection of three of the other structural isomers. A shortcoming of using photoionization efficiency curves for isomer separation is that these curves, particularly for the species in question, look exceedingly similar which makes disentangling a total ion yield comprising multiple isomers problematic when the isomers have ionization offsets close to each other. A more sensitive method of isomer detection in mass spectrometric studies is using the threshold photoelectron spectroscopic method (TPES) which offers a more unique fingerprint to identify molecules in complex media as a TPES is more unique to the molecule than its total ionization efficiency curve. Isomer differentiation is achieved by simultaneously recording the vibronic structures of all isomers present in a single mass signature. In this work, we have employed a microwave discharge flowtube reactor to produce C$_3$H$_3$ radicals in a continuous and stable manner. The flowtube is coupled to a synchrotron VUV radiation source at the French national facility SOLEIL, and the mass selected photoelectron spectra were recorded to identify and disentangle all isomeric C$_6$H$_6$ reaction products via i$^2$PEPICO techniques. Besides the primary product of interest, benzene, we also detect most of the reaction intermediates formed from head to head, tail to tail collsion of the propargyl radicals. 
       
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