I2 molecular elimination in single-photon dissociation of CH_2I_2 at 248 nm by using cavity ring-down absorption spectroscopy

摘要

Following single-photon dissociation of CH_2I_2 at 248 nm, I_2 molecular elimination is detected by using cavity ring-down absorption spectroscopy. The technique comprises two laser beams propagating in a perpendicular configuration, in which a tunable laser beam along the axis of the ring-down cell probes the I2 fragment in the B 3 _ou + - X 1 g + transition. The nascent vibrational populations for v 0, 1, and 2 levels are obtained with a population ratio of 1:(0.65 0.10):(0.30 0.05), corresponding to a Boltzmann-like vibrational temperature of 544 73 K. The quantum yield of the ground state I_2 elimination reaction is determined to be 0.0040 0.0025. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state CH_2I_2 via internal conversion, followed by asynchronous three-center dissociation. A positive temperature effect supports the proposed mechanism.