Direct measurement of near-threshold rate constants for unimolecular dissociation of CF3I after IR multiphoton excitation

Excited-state populations of CF₃I after IR multiphoton excitation were monitored by time-resolved hot-band UV absorption spectroscopy. Using a calibration of the spectrum by shock-wave experiments, the absorption changes during the laser pulse are analyzed with respect to excited-state populations and dissociation at higher excitation energies. Dissociation of molecules near threshold is detected under collision-free conditions by absorption changes after the laser pulse. At higher pressures, absorption signals after the pulse are markedly influenced by energy transfer between excited and cold molecules. The measured dissociation rate constants near threshold are consistent with statistical calculations of k(E,J), showing pronounced rotational dependence.