Cavity Ring-Down Absorption Spectroscopy: Optical Characterization of ICl Product in Photodissociation of CH 2 ICl at 248 nm.

Iodine monochloride (ICl) elimination from one-photon dissociation of CH2 ICl at 248 nm is monitored by cavity ring-down absorption spectroscopy (CRDS). The spectrum of ICl is acquired in the transition of B3 Π0 ← X1 Σ+ and is confirmed to result from a primary photodissociation, that is, CH2 ICl + hν → CH2 + ICl. The vibrational population ratio is determined with the aid of spectral simulation to be 1:(0.36 ± 0.10):(0.11 ± 0.05) for the vibrational levels ν = 0, 1, and 2 in the ground electronic state, corresponding to a Boltzmann-like vibrational temperature of 535 ± 69 K. The quantum yield of the ICl molecular channel for the reaction is obtained to be 0.052 ± 0.026 using a relative method in which the scheme CH2 Br2 → CH2 + Br2 is adopted as the reference reaction. The ICl product contributed by the secondary collisions is minimized such that its quantum yield obtained is not overestimated. With the aid of the CCSD(T)//B3LYP/MIDI! level of theory, the ICl elimination from CH2 ICl is evaluated to follow three pathways via either (1) a three-center transition state or (2) two isomerization transition states. However, the three-center concerted mechanism is verified to be unfavorable.