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

We interrogated C 7 H produced from reactions C 4 + C 3 H 2 /C 4 H + C 3 H → C 7 H + H using both translational and photoionization spectroscopy. Reactants C 3 H, C 3 H 2, C 4, and C 4 H were synthesized in two crossed beams of 1% C 2 H 2 /He ignited by pulsed high-voltage discharge. The individual contributions of reactions C 4 + C 3 H 2 and C 4 H + C 3 H to product C 7 H 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 C 7 H were unraveled. C 7 H 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-C 7 H + . The quantum-chemical calculations indicate that the associations of C 4 with C 3 H 2 and C 4 H with C 3 H incur no entrance barriers, and the most favorable exit channel leads to product l-C 7 H + H. It is the first time demonstrating that C 7 H is producible from reactions 1,3 C 4 + 1 C 3 H 2 and 2 C 4 H + 2 C 3 H on the lowest-lying singlet and triplet potential energy surfaces of 1,3 C 7 H 2 . This work implies that the reactions of C 4 + C 3 H 2 and C 4 H + C 3 H might have contributions to interstellar C 7 H to some extent as compared with the C + C 6 H 2 reaction commonly adopted in an astrochemical model.