Direct observation of intramolecular vibrational energy redistribution of selectively excited CH2I2 and C3H5I molecules in solution

A time resolved study of intramolecular vibrational energy redistribution (IVR) in competition with intermolecular vibrational energy transfer (VET) of vibrationally excited methylene iodide (CH2I2) and allyl iodide (C3H5I) in solution is reported. Near IR-laser pulses between 1.7 and 2.4 mum selectively excited the molecules in the C-H-stretch overtone or combination bands and transient absorption at 400 nm monitored IVR on different ps timescales as well as VET to the solvent on a longer timescale. The transient absorption was calibrated against thermal absorption spectra at high temperatures measured in shock waves. With a simple model we have been able to determine global rate coefficients tau (IVR) for the intramolecular equilibration of vibrational energy in the range 12-22 ps for CH2I2 and 2-3 ps for C3H5I. The picture that emerges from these studies is that tau (IVR) for methylene iodide is only weakly dependent upon the excitation energy and the excited (zeroth order) mode and even less dependent on the solvent. The fact that it depends on the nature of the solvent at all supports our conclusion that IVR for CH2I2 is weakly solvent assisted, as opposed to C3H5I. In the case of allyl iodide we find considerably smaller values for tau (IVR) and a much faster decay due to VET with almost no dependence on the excitation energy and the solvent. Since IVR rates for both molecules do not scale at all with calculated densities of states, we conclude that the rate of intramolecular energy transfer in solution for both molecules in all solvents investigated in this study is still dominated by specific intramolecular interactions causing non-statistical IVR.