Study on Formation of Interstellar C₇H from Reactions C₄ + C₃H₂ and C₄H + C₃H

We interrogated C₇H produced from reactions C₄ + C₃H₂/C₄H + C₃H → C₇H + H using both translational and photoionization spectroscopy. Reactants C₃H, C₃H₂, C₄, and C₄H were synthesized in two crossed beams of 1% C₂H₂/He ignited by pulsed high-voltage discharge. The individual contributions of reactions C₄ + C₃H₂ and C₄H + C₃H to product C₇H were evaluated as 17:83 from reactant concentrations in both molecular beams. The translational energy distribution, the angular distribution, and the photoionization efficiency curve of product C₇H were unraveled. C₇H was identified as the most stable linear isomer by its photoionization efficiency curve that features two ionization thresholds corresponding to separate transitions to singlet and triplet states of l-C₇H⁺. The quantum-chemical calculations indicate that the associations of C₄ with C₃H₂ and C₄H with C₃H incur no entrance barriers, and the most favorable exit channel leads to product l-C₇H + H. It is the first time demonstrating that C₇H is producible from reactions ^1,3C₄ + ¹C₃H₂ and ²C₄H + ²C₃H on the lowest-lying singlet and triplet potential energy surfaces of ^1,3C₇H₂. This work implies that the reactions of C₄ + C₃H₂ and C₄H + C₃H might have contributions to interstellar C₇H to some extent as compared with the C + C₆H₂ reaction commonly adopted in an astrochemical model.