193Â nm photolysis of CHCl3: Probe of the CH product by CRDS

The CH radical production induced by 193 nm two-photon photolysis of CHCl3 has been measured for the first time via the cavity ring-down absorption spectroscopy of its A-X bands, using a commercial nanosecond pulsed dye laser. The range of pressure and laser intensity, as well as the time window detection, have been carefully chosen to ensure a constant CH number density during the measurement and to avoid post-photolysis reactivity. Internal energy distribution of the CH(X2II) fragment has been derived from population distribution simulations, leading to an average vibrational temperature Tvib = 1900 ± 50 K and rotational temperature Trot = 300 ± 20 K. Two competing mechanisms can be invoked for the CH production channel: either two-photon absorption via resonant excited states of CHCl3 leading to dissociation of excited CHCl3, or two-photon sequential dissociation via the formation of the vibrationally excited CHCl2 fragment. The latter mechanism is proposed to be the prominent process for CH formation.