Fluorescence lifetimes and predissociation processes in the (B)over-tilde(2)A ' state of CCH

Fluorescence lifetimes have been measured under supersonic jet conditions f or various N, K-rotational levels of three vibrational levels (T, T+nu(2), and T+nu(3)) of the (B) over tilde (2)A' state of CCH. It was found that th e lifetimes of levels T+nu(2) and T+nu(3) are significantly shorter than th at of level T. Furthermore, the lifetimes vary in a complicated way from on e rovibronic level to another, indicating the presence of local perturbatio ns. 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 diss ociation process, CCH ((X) over tilde (2)Sigma(+))--> C-2(X (1)Sigma(g)(+)) +H(S-2(1/2)). In this work, fluorescence excitation spectra of photofragmen t C-2 via D (1)Sigma(u)(+)<-- X (1)Sigma(g)(+) and d (3)Pi(g)<-- a (3)Pi(u) have been recorded to determine the predissociation processes of the (B) o ver tilde state. Assuming no barrier is present in the dissociation process of the (X) over tilde and (A) over tilde states, the upper bound of D-0(0) (CC-H) was estimated as 39 388 +/- 7 cm(-1)(similar or equal to 112.62 kcal /mol). Hence, the lifetime quenching phenomenon observed in the (B) over ti lde state is primarily due to predissociation via the dissociative continuu m of either the (X) over tilde or the (A) over tilde state of CCH. (C) 2000 American Institute of Physics. [S0021-9606(00)01217-4].