DECOMPOSITION OF HIGHLY VIBRATIONALLY EXCITED CDCL3

Decomposition of highly vibrationally excited CDCl3 was studied in the time domain by measuring laser-induced fluorescence from one of the d ecomposition products CCl2 or by observing luminescence from CCl2 radi cal fragments produced in the A approximately (1B1) state following IR excitation. It is shown that highly vibrationally excited CDCl3 can b e made via two different routes: simple optical absorption of an incid ent (CO2)-C-13 laser pulse or through collision-moderated energy pooli ng coupled with photon absorption. Higher fluence measurements are con sistent with the former and support previous claims that the infrared multiphoton decomposition probability for CDCl3 is pressure independen t. At a lower fluence the vibrational up-pumping mechanism apparently relies heavily upon collisions, which supports other claims that the d ecomposition probability is pressure dependent. The results of the pre sent work reconcile these previous disparate claims. Furthermore, ther e is an indication that the vibrational energy transferred during a co llision could be much larger than collision-induced dipole selection r ules would allow. In some of the experiments, electronically excited C Cl2 fragments are created but only through post-laser-pulse collisions that appear to involve exchanges of very large amounts of vibrational energy. By contrast, no electronically excited CCl2 is produced follo wing excitation with the CO2 laser alone. The experiments also corrobo rate an earlier prediction that the extent of reaction is governed by adiabatic expansion of the centrally heated zone, even at pressures as low as 0.66 kPa, rather than by diffusion, as is commonly believed.