Faculté des sciences

Dissociative electron attachment to methylacetylene and dimethylacetylene: Symmetry versus proximity

Janečková, Radmila ; May, Olivier ; Fedor, Juraj

In: Physical Review A - Atomic, Molecular, and Optical Physics, 2012, vol. 86, no. 05, p. 052702

We have measured absolute dissociative electron attachment (DEA) cross sections in methylacetylene (propyne, C₃H₄) and dimethylacetylene (but-2-yne, C₄H₆). The main feature in the low-energy DEA spectrum is a π* shape resonance giving rise to fragments at 3.4 eV in C₃H₄ and 4.0 eV in C₄H₆. The process C₃H₄+e→ C₃H₃ −+H proceeds via abstraction... Plus

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    Summary
    We have measured absolute dissociative electron attachment (DEA) cross sections in methylacetylene (propyne, C₃H₄) and dimethylacetylene (but-2-yne, C₄H₆). The main feature in the low-energy DEA spectrum is a π* shape resonance giving rise to fragments at 3.4 eV in C₃H₄ and 4.0 eV in C₄H₆. The process C₃H₄+e→ C₃H₃  −+H proceeds via abstraction of the acetylenic hydrogen which is mediated by effective vibronic coupling. The abstraction of methyl group hydrogen, which does not require symmetry lowering of the transient negative ion, was not observed; the spatial separation of the bond from the resonance decreases the cross section dramatically. The presence of the methyl group further influences the DEA cross sections via change of the resonance's position and via blocking one or both of the DEA active sites in acetylene. No cleavage of the C-methyl bond has been observed, and reasons for this effect are discussed. Additionally, several higher-lying resonances (6 to 15 eV) leading to production of an H⁻ fragment were observed and assigned.