DYNAMICS OF UNIMOLECULAR REACTIONS INDUCED BY MONOCHROMATIC IR RADIATION - EXPERIMENT AND THEORY FOR CNFMHKI-]CNFMHK-LIMITED, DOPPLER-LIMITED AND UNCERTAINTY-LIMITED TIME RESOLUTION OF IODINE-ATOM IR ABSORPTION(I PROBED WITH HYPERFINE)

Coherent multiphoton excitation of polyatomic molecules with pulsed CO 2 lasers leads to unimolecular reactions induced by monochromatic infr ared radiation (URIMIR). We report a detailed study of the dynamics of dissociation of trifluoroiodomethane (CF3I), 1,1,1,2-tetrafluoro-2-io doethane (CF3CHFI) and pentafluoroiodobenzene (C6F5I). The primary dis sociation after ro-vibrational excitation in the electronic ground sta te results in iodine atoms I(P-2(3/2)), which are detected by diode la ser IR absorption on the (P-2(3/2)-P-2(1/2)) magnetic dipole transitio n with about 1 MHz frequency resolution and up to 1 ns time resolution , essentially bounded by the uncertainty principle. This allows us to detect the product-state distribution over nuclear hyperfine levels in I atoms, and product translational-energy distributions from Doppler lineshapes combined with quantitative, time-resolved kinetic analysis under conditions of irradiation with shape-controlled CO2 laser pulses of well defined fluence and intensity. The kinetic results for absolu te rates are analysed in terms of the laser chemical rate coefficient k(I)(st) and compared to theoretical calculations based on the case B/ C master equation including non-linear intensity effects, which are fo und to be important only for CF3I. The results for relative rates are analysed in terms of a simple theoretical model for the centre-of-mass product translational-energy distribution P(E-t). The results are dis cussed in relation to the foundations of IR laser chemistry.