Fluorescence lifetimes and predissociation processes in the B̃ 2A′ state of CCH

Abstract

Fluorescence lifetimes have been measured under supersonic jet conditions for various N, K-rotational levels of three vibrational levels (T, T+ν2, and T+ν3) of the B̃ 2A′ state of CCH. It was found that the lifetimes of levels T+ν2 and T+ν3 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 (X̃ 2Σ+)→C2(X 1Σg+)+H(2S1/2). In this work, fluorescence excitation spectra of photofragment C2 via D 1Σu+←X 1Σg+ and d 3Πg←a 3Πu have been recorded to determine the predissociation processes of the B̃ state. Assuming no barrier is present in the dissociation process of the X̃ and à states, the upper bound of D00(CC–H) was estimated as 39 388±7 cm−1(≃112.62 kcal/mol). Hence, the lifetime quenching phenomenon observed in the B̃ state is primarily due to predissociation via the dissociative continuum of either the X̃ or the à state of CCH.