I2 molecular elimination in single-photon dissociation of CH2I2 at 248 nm by using cavity ring-down absorption spectroscopy

Following single-photon dissociation of CH2I2 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 I-2 fragment in the B-3 Pi(vertical bar)(ou) - X-1 Sigma(vertical bar)(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 CH2I2 via internal conversion, followed by asynchronous three-center dissociation. A positive temperature effect supports the proposed mechanism. (C) 2011 American Institute of Physics. [doi:10.1063/1.3523571]