Predissociation of the acetyleneA 1Austate and its mechanism

The fluorescence excitation spectrum, the MPI spectrum, and the absorption spectrum of acetylene were observed for the à 1Au ←X̃ 1∑+g band system in a static gas cell and in a supersonic jet. A sudden and drastic decrease in fluorescence quantum yield, Φf , was found between the V 4 K2 (46 339 cm−1 ) and 21 V 3 K0 (46 673 cm−1 ) sublevels. The decrease in Φf is concluded to be due to the predissociation into C2 H+H. J dependence of Φf was found for the V 4 K1 vibronic sublevel, while it is absent for the level lying above V4 K1 . From the observed J dependence, the predissociation mechanism was concluded to be vibrational predissociation by tunneling through a potential barrier.