Identifying isomers of peroxy radicals in the gas phase The two isomers of propylperoxy radical to mimic chemical reactions in the atmosphere, in which the 1-CHO and 2-CHO, together with propylperxoy radical was generated through the reaction of 3 7 2 3 7 2 the propyl radical with oxygen, initiated with fluorine atoms in their individual rotamers, are identified a CH/O/He mixture. Two kinds of CH cations (m/z = 43),+ originating from direct photoionization of the propyl radical (CH)3 8 2 3 7 and assigned by threshold photoelectron 3 7 spectroscopy with the aid of high-level and from dissociative photoionization of the propylperoxy radical (CHO) whose cation is not stable, have been observed in the3 27 theoretical computations, from which their experiments and can clearly be separated in the photoionization accurate adiabatic ionization energies are time-of-flight (TOF) mass spectra and ion images through their derived. This study paves the way to probing different ion kinetic energies, as seen in Figure 1. elusive peroxy radicals and their isomers FIGURE 1 in advanced mass spectrometry analysis of combustion and atmospheric reactions. Peroxy radicals (RO) are key reaction intermediates in the2 low temperature oxidation of organic compounds and play essential roles in combustion processes and in atmospheric chemistry. In the atmosphere, they are formed by the reaction of oxygen with alkyl radicals (R), via a three-body collision reaction R + O + M → RO + M, where M represents a third species2 2 removing the excess internal energy of the nascent peroxy radicals by collisions. Then, peroxy radicals can react with various species like NO (NO and NO), HO (OH and HO) and2 2 x x other RO, as well as performing self-reaction and unimolecular2 reaction, and their reactivity can lead to the formation of a series of secondary pollutants such as ozone and secondary organic aerosols, with a high impact on the atmospheric chemical content and human health. The reaction mechanisms involving peroxy radicals are closely linked to their specific structures, The mass-selected threshold photoelectron spectrum (TPES) which become progressively complex with increasing mass, and corresponding only to the propylperoxy radical was then usually contain several structural isomers, plus their rotamers. measured in coincidence by scanning the photon energy as However, due to their complexity and in particular the instability shown in Figure 2. A vibrational fine structure can clearly be of their cations, probing the isomers of peroxy radicals is still very observed in the 9–11 eV energy range of the TPES benefiting challenging, limiting our understanding of chemical reactions of the high energy resolution of the DESIRS beamline and of networks in which they might be involved. the iPEPICO setup DELICIOUS 3 available on this beamline.2 EXPERIMENTAL RESULTS In order to identify the detailed structures of the propylperoxy Very recently, the above-described challenge of identifying radical, high-level theoretical calculations were performed isomers of peroxy radicals has been successfully tackled on the and predicted that, in the flow tube, the propylperoxy radical DESIRS beamline at SOLEIL by an international consortium of should be produced as two isomers, 1-CHO and 2-CHO,3 32 27 7 researchers from Hefei Institutes of Physical Science (Chinese together with their individual rotamers, i.e. the GG, GT, TG,2 2 21 1 1 Academy of Sciences), Université Gustave Eiffel, CNRS- TT and G’G of 1-CHO, and the G and T of 2-CHO, as3 32 2 2 21 1 7 7 Bordeaux, CNRS-Lille and the DESIRS beamline group, by shown in Figure 3. The stable structure of these rotamers and using double imaging photoelectron photoion coincidence their corresponding cations, their individual energetic levels and (iPEPICO) spectroscopy, together with high level theoretical2 potential energy curves involved in their photoionization, have computations. A microwave discharge flow tube was employed also been theoretically calculated and discussed. 60