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) o f vibrationally excited methylene iodide (CH2I2) and allyl iodide (C3H5I) i n solution is reported. Near IR-laser pulses between 1.7 and 2.4 mum select ively excited the molecules in the C-H-stretch overtone or combination band s and transient absorption at 400 nm monitored IVR on different ps timescal es as well as VET to the solvent on a longer timescale. The transient absor ption was calibrated against thermal absorption spectra at high temperature s measured in shock waves. With a simple model we have been able to determi ne 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 methylen e iodide is only weakly dependent upon the excitation energy and the excite d (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 tha t IVR for CH2I2 is weakly solvent assisted, as opposed to C3H5I. In the cas e of allyl iodide we find considerably smaller values for tau (IVR) and a m uch faster decay due to VET with almost no dependence on the excitation ene rgy 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 intramol ecular energy transfer in solution for both molecules in all solvents inves tigated in this study is still dominated by specific intramolecular interac tions causing non-statistical IVR.