The formation of C2H− is observed in two broad resonance bands when C2H2 is irradiated with vuv light. The higher‐energy band has partially resolved structure, approximately linear pressure dependence, and a threshold at 16.335±0.021 eV. It is attributed to photoion‐pair formation (C2H−+H+) consequent upon predissociation of one or more Rydberg states. This threshold, together with IP(H) and EA(C2H), gives D0(HCC–H)≤5.706±0.022 eV≡131.6±0.5 kcal/mol, or ΔH0f0 (C2H)≤134.5±0.5 kcal/mol. The lower‐energy band has an approximately quadratic pressure dependence and curved step‐like structure. It is attributed to photoelectron‐induced dissociative attachment mediated by a πg shape resonance. The threshold, at 878.5±2.0 Å, corresponds to a photoelectron energy of 2.715±0.032 eV. This threshold combined with EA(C2H)=2.969±0.010 eV, yields D0(HCC–H)≤5.684±0.033 eV≡131.1±0.7 kcal/mol, or ΔH0f0 (C2H)=134.0±0.7 kcal/mol.

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