Formations of C₆H from reactions C₃ + C₃H₂ and C₃H + C₃H and of C₈H from reactions C₄ + C₄H₂ and C₄H + C₄H

We interrogated C₆H and C₈H produced separately from the reactions C₃ + C₃H₂/C₃H + C₃H/C₃H₂ + C₃ → C₆H + H and C₄ + C₄H₂/C₄H + C₄H/C₄H₂ + C₄ → C₈H + H using product translational and photoionization spectroscopy. Individual contributions of the three reactions to the product C₆H or C₈H were evaluated with reactant concentrations. Translational-energy distributions, angular distributions, and photoionization efficiency curves of products C₆H and C₈H were unraveled. The product C₆H (C₈H) was recognized as the most stable linear isomer by comparing its photoionization efficiency curve with that of l-C₆H (l-C₈H), produced exclusively from the reaction C₂ + C₄H₂ → l-C₆H + H (C₂ + C₆H₂ → l-C₈H + H). The ionization threshold after deconvolution was determined to be 9.3 ± 0.1 eV for l-C₆H and 8.9 ± 0.1 eV for l-C₈H, which is in good agreement with theoretical values. Quantum-chemical calculations indicate that the reactions of C₃ + C₃H₂ and C₃H + C₃H (C₄ + C₄H₂ and C₄H + C₄H) incur no energy barriers that lie above the corresponding reactant and the most stable product l-C₆H (l-C₈H) with H on the lower-lying potential-energy surfaces. The theoretical calculation is in accord with the experimental observation. This work implies that the reactions of C₃ + C₃H₂/C₃H + C₃H and C₄ + C₄H₂/C₄H + C₄H need to be taken into account for the formation of interstellar C₆H and C₈H, respectively.