Dynamics of unimolecular reactions induced by monochromatic IR radiation: experiment and theory for CnFmHkI → CnFmHk+ I probed with hyperfine-, doppler- and uncertainty-limited time resolution of iodine-atom IR absorption

Abstract

Coherent multiphoton excitation of polyatomic molecules with pulsed CO₂ lasers leads to unimolecular reactions induced by monochromatic infrared radiation (URIMIR). We report a detailed study of the dynamics of dissociation of trifluoroiodomethane (CF₃I), 1,1,1,2-tetrafluoro-2-iodoethane (CF₃CHFI) and pentafluoroiodobenzene (C₆F₅I). The primary dissociation after ro-vibrational excitation in the electronic ground state results in iodine atoms I(²P_3/2), which are detected by diode laser IR absorption on the (²P_3/2–²P_1/2) magnetic dipole transition 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 absolute 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 found 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 discussed in relation to the foundations of IR laser chemistry.