Fluorescence lifetimes and predissociation processes in the B̃²A′ state of CCH

Fluorescence lifetimes have been measured under supersonic jet conditions for various N, K-rotational levels of three vibrational levels (T, T + v₂, and T + v₃) of the B̃²A′ state of CCH. It was found that the lifetimes of levels T + v₂ and T + v₃ are significantly shorter than that of level T. Furthermore, the lifetimes vary in a complicated way from one rovibronic level to another, indicating the presence of local perturbations. The previously reported CC-H bond energy ranges from 95.6 to 116.3 kcal/mol, which leads to a large uncertainty in assigning the onset of the dissociation process, CCH (X̃²∑⁺)→C₂(X ¹∑_g⁺) + H(²S_1/2). In this work, fluorescence excitation spectra of photofragment C₂ via D ¹∑←X¹∑_g⁺ and d ³Π_g←a ³Π_u have been recorded to determine the predissociation processes of the B̃ state. Assuming no barrier is present in the dissociation process of the X and A states, the upper bound of D₀⁰(CC-H) was estimated as 39 388±7 cm^-1(≃112.62kcal/mol). Hence, the lifetime quenching phenomenon observed in the B state is primarily due to predissociation via the dissociative continuum of either the X̃ or the à state of CCH.