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

We interrogated C7H produced from reactions C-4 + C3H2/C4H + C3H -> C7H + H using both translational and photoionization spectroscopy. Reactants C3H, C3H2, C-4, and C4H were synthesized in two crossed beams of 1% C2H2/He ignited by pulsed high-voltage discharge. The individual contributions of reactions C-4 + 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 C-4 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 C-1,3(4) + (C3H2)-C-1 and (C4H)-C-2 + (C3H)-C-2 on the lowest-lying singlet and triplet potential energy surfaces of (C7H2)-C-1,3. This work implies that the reactions of C-4 + 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.