Formation of Polyynes C₄H₂, C₆H₂, C₈H₂, and C₁₀H₂ from Reactions of C₂H, C₄H, C₆H, and C₈H Radicals with C₂H₂

Abstract

Some of the polyynes (HC2n+2H, 1 ≤ n ≤ 4) are observable in planetary atmospheres, interstellar space, and flames. Polyynes are proposed to play an important role in synthesis of large carbonaceous molecules. We explore the dynamics of reactions of C2nH (n = 1-4) radicals with C2H2 by interrogating time-of-flight spectra and photoionization efficiency spectra of products C2n+2H2. The reactions of n = 2-4 were investigated for the first time. The translational energy release is biased to low energy but extends to the energetic limit of product HC2n+2H + H, corresponding to a fraction of 0.34-0.36 on translational energy. Product C2n+2H2 has a deconvoluted ionization threshold in good agreement with the ionization energy of polyynes. The quantum chemical calculations support the experimental observations. This work verifies that the title reaction is an important source for formation of polyynes that have been observed in interstellar/circumstellar media and combustion processes.