Real-time probing of intramolecular vibrational energy redistribution and intermolecular vibrational energy transfer of selectively excited CH2I2 molecules in solution

Competition between intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of excited methylene i odide (CH2I2) in solution has been measured in real time. After excitation of the C-H- stretch overtone and C-H- stretch containing combination bands of CH2I2 between 1.7 and 2.4 mum an increase followed by a decrease in the transient electronic absorption at 400 nm has been monitored. The transient absorption has been attributed to vibrational energy flow from the initial ly excited degrees of freedom to vibrational states with larger Franck-Cond on (FC) factors for the electronic transition (long wavelength wing) and en ergy loss due to energy transfer to the solvent. A model based upon the dep endence of the electronic absorption on the internal energy <E > of CH2I2 h as been used to determine the times for intramolecular vibrational energy r edistribution and intermolecular energy transfer to the solvent. in the sim plest version of our model the internal energy of the molecule probed by th e population of the FC-active modes rises and decays exponentially on a pic osecond (ps) time scale, which reflects the initial intramolecular vibratio nal energy redistribution and the subsequent energy transfer to the solvent . This simple approach was able to accurately describe the measured transie nt absorption for all solvents and excitation wavelengths. Overall time con stants for IVR have been found to be on the order of 9-10 ps, almost indepe ndent of the excitation wavelength, the excited modes, and the solvent. In contrast, energy transfer to the solvent takes significantly longer. Overal l time constants for VET have been determined in the range between 60 and 1 20 ps depending on the solvent, the excitation energy, but not on the mode which was initially excited.